Add ALPN support to WiFiClientSecure (#5633)

This adds a function to WiFiClientSecure to set the ALPN protocol.

This is required for an MQTT client to connect to AWS IoT when using an AWS Custom Authorizer, as described here.

Example code snippet:

...
WiFiClientSecure wiFiClient;

// ALPN protocol, needed with AWS custom authorizer
const char *aws_protos[] = {"mqtt", NULL};

void setup() {
  wiFiClient.setCACert(AWSCAPEM);
  wiFiClient.setAlpnProtocols(aws_protos);
}
...

libraries/WiFiClientSecure/keywords.txt

@@ -29,6 +29,7 @@ connected	KEYWORD2
 setCACert	KEYWORD2
 setCertificate	KEYWORD2
 setPrivateKey	KEYWORD2
+setAlpnProtocols	KEYWORD2
 
 #######################################
 # Constants (LITERAL1)