feat(modem): Add mqtt example in AT-only mode

Similar to pppos-client, but without PPP mode.
This uses standard mqtt client and a loopback service that forwards the
TCP traffic from localhost to the modem and vice-versa.
(next step is to create a dedicated tcp-transport layer for modem)
Console example uses DCE commandable to demontrate how to handle URC

components/esp_modem/examples/modem_tcp_client/main/Kconfig.projbuild

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+menu "Example Configuration"
+
+    choice EXAMPLE_MODEM_DEVICE
+        prompt "Choose supported modem device (DCE)"
+        default EXAMPLE_MODEM_DEVICE_BG96
+        help
+            Select modem device connected to the ESP DTE.
+        config EXAMPLE_MODEM_DEVICE_SIM800
+            bool "SIM800"
+            help
+                SIMCom SIM800L is a GSM/GPRS module.
+                It supports Quad-band 850/900/1800/1900MHz.
+        config EXAMPLE_MODEM_DEVICE_BG96
+            bool "BG96"
+            help
+                Quectel BG96 is a series of LTE Cat M1/Cat NB1/EGPRS module.
+        config EXAMPLE_MODEM_DEVICE_SIM7600
+            bool "SIM7600"
+            help
+                SIM7600 is Multi-Band LTE-TDD/LTE-FDD/HSPA+ and GSM/GPRS/EDGE module
+    endchoice
+
+    config EXAMPLE_MODEM_APN
+        string "Set MODEM APN"
+        default "internet"
+        help
+            Set APN (Access Point Name), a logical name to choose data network
+
+    config EXAMPLE_MODEM_PPP_AUTH_USERNAME
+        string "Set username for authentication"
+        default "espressif"
+        depends on !EXAMPLE_MODEM_PPP_AUTH_NONE
+        help
+            Set username for PPP Authentication.
+
+    config EXAMPLE_MODEM_PPP_AUTH_PASSWORD
+        string "Set password for authentication"
+        default "esp32"
+        depends on !EXAMPLE_MODEM_PPP_AUTH_NONE
+        help
+            Set password for PPP Authentication.
+
+    config EXAMPLE_MODEM_PPP_AUTH_NONE
+        bool "Skip PPP authentication"
+        default n
+        help
+            Set to true for the PPP client to skip authentication
+
+    config EXAMPLE_SEND_MSG
+        bool "Short message (SMS)"
+        default n
+        help
+            Select this, the modem will send a short message before power off.
+
+    if EXAMPLE_SEND_MSG
+        config EXAMPLE_SEND_MSG_PEER_PHONE_NUMBER
+            string "Peer Phone Number (with area code)"
+            default "+8610086"
+            help
+                Enter the peer phone number that you want to send message to.
+    endif
+
+    config EXAMPLE_NEED_SIM_PIN
+        bool "SIM PIN needed"
+        default n
+        help
+            Enable to set SIM PIN before starting the example
+
+    config EXAMPLE_SIM_PIN
+        string "Set SIM PIN"
+        default "1234"
+        depends on EXAMPLE_NEED_SIM_PIN
+        help
+            Pin to unlock the SIM
+
+    menu "UART Configuration"
+        config EXAMPLE_MODEM_UART_TX_PIN
+            int "TXD Pin Number"
+            default 25
+            range 0 31
+            help
+                Pin number of UART TX.
+
+        config EXAMPLE_MODEM_UART_RX_PIN
+            int "RXD Pin Number"
+            default 26
+            range 0 31
+            help
+                Pin number of UART RX.
+
+        config EXAMPLE_MODEM_UART_RTS_PIN
+            int "RTS Pin Number"
+            default 27
+            range 0 31
+            help
+                Pin number of UART RTS.
+
+        config EXAMPLE_MODEM_UART_CTS_PIN
+            int "CTS Pin Number"
+            default 23
+            range 0 31
+            help
+                Pin number of UART CTS.
+
+        config EXAMPLE_MODEM_UART_EVENT_TASK_STACK_SIZE
+            int "UART Event Task Stack Size"
+            range 2000 6000
+            default 2048
+            help
+                Stack size of UART event task.
+
+        config EXAMPLE_MODEM_UART_EVENT_TASK_PRIORITY
+            int "UART Event Task Priority"
+            range 3 22
+            default 5
+            help
+                Priority of UART event task.
+
+        config EXAMPLE_MODEM_UART_EVENT_QUEUE_SIZE
+            int "UART Event Queue Size"
+            range 10 40
+            default 30
+            help
+                Length of UART event queue.
+
+        config EXAMPLE_MODEM_UART_PATTERN_QUEUE_SIZE
+            int "UART Pattern Queue Size"
+            range 10 40
+            default 20
+            help
+                Length of UART pattern queue.
+
+        config EXAMPLE_MODEM_UART_TX_BUFFER_SIZE
+            int "UART TX Buffer Size"
+            range 256 2048
+            default 512
+            help
+                Buffer size of UART TX buffer.
+
+        config EXAMPLE_MODEM_UART_RX_BUFFER_SIZE
+            int "UART RX Buffer Size"
+            range 256 2048
+            default 1024
+            help
+                Buffer size of UART RX buffer.
+    endmenu
+
+endmenu