[cli-tcp] add TLS mode to TCP CLI tool and TLS callbacks to TCP API (#8257)

This commit adds support for TLS in the TCP CLI tool, along with a
test of TLS functionality.

The main purpose of this is to provide an example of how to use
mbedTLS with TCPlp to run TLS. I felt it is important to have an
example because I expect that many applications that use TCP,
particularly those in which a Thread device exchanges data with a
device outside the Thread mesh, will need to run TLS over TCP for
security reasons.

A secondary benefit is to add TCP functionality to the TCP CLI
tool. Additionally, by having the example code be part of a "real"
application (the CLI tool), there is less risk of the code going stale
as OpenThread evolves.

I added an "expects" test for the TLS functionality in the CLI tool. I
also ran the code on two Nordic NRF52840-DK boards to make sure it
works.

The performance impact of TLS appears to be small. When using TCP
without TLS, I am able to achieve ~80 kb/s goodput between two
NRF52840-DK boards. With TLS in the same setup, I am able to achieve
~70-75 kb/s goodput.

third_party/mbedtls/mbedtls-config.h

@@ -86,6 +86,9 @@
 
 #if OPENTHREAD_CONFIG_COAP_SECURE_API_ENABLE
 #define MBEDTLS_KEY_EXCHANGE_PSK_ENABLED
+#endif
+
+#if OPENTHREAD_CONFIG_COAP_SECURE_API_ENABLE || OPENTHREAD_CONFIG_TLS_ENABLE
 #define MBEDTLS_KEY_EXCHANGE_ECDHE_ECDSA_ENABLED
 #endif