Add certificate bundle capability to WiFiClientSecure (#6106)

* Add certificate bundle capability to WiFiClientSecure

Enable usage of the ESP32 IDF's certificate bundle for WiFiClientSecure connections.

Adds the ability to load a bundle or root certificates and use them for authenticating SSL servers.

Based on work from Onno-Dirkzwager, Duckle29, kubo6472, meltdown03, kinafu and others.

See also:
- https://docs.espressif.com/projects/esp-idf/en/latest/esp32/api-reference/protocols/esp_crt_bundle.html
- https://github.com/espressif/arduino-esp32/issues/3646
- libraries/WiFiClientSecure/README.md

* Fix build issues

* Clean up old bundle index when NULL bundle is attached

libraries/WiFiClientSecure/src/WiFiClientSecure.h

@@ -40,6 +40,7 @@ protected:
     const char *_pskIdent; // identity for PSK cipher suites
     const char *_psKey; // key in hex for PSK cipher suites
     const char **_alpn_protos;
+    bool _use_ca_bundle;
 
 public:
     WiFiClientSecure *next;
@@ -70,6 +71,7 @@ public:
     void setCertificate(const char *client_ca);
     void setPrivateKey (const char *private_key);
     bool loadCACert(Stream& stream, size_t size);
+    void setCACertBundle(const uint8_t * bundle);
     bool loadCertificate(Stream& stream, size_t size);
     bool loadPrivateKey(Stream& stream, size_t size);
     bool verify(const char* fingerprint, const char* domain_name);