gbridge: controllers: add a tls controller

[cfriedt]

This change is required to support cfriedt/greybus-for-zephyr#25, in which TLS support was added to greybus-for-zephyr.

In this context, gbridge is the client and Zephyr (or some other process / device) is the server, "certificate" is synonymous with "public key", etc.

In this ticket, we would like to enable TLS support for 2 possible configurations:

1. no client verification

• gbridge verifies the server only using a CA certificate
• the CA certificate is only required on the client-side if using a self-signed certificate on the server
• this configuration uses standard TLS 1.2 to perform one-way authentication (client authenticates server)
• this is the least secure configuration

2. required client verification

• gbridge verifies the server using a CA certificate
• gbridge provides credentials to the server upon request
• this configuration uses standard TLS 1.2 to perform mutual authentication between the client and server
• this is the most secure configuration

In both of the above cases, usage of the standard TLS 1.2 protocol ensures that industry standard symmetric key negotiation and encryption practices are used after authentication is performed.

OpenSSL has s_server.c and s_client.c as examples with server and client documentation.