[flagd-RPC] TLS custom cert support

[toddbaert]

Support custom certs for RPC flagd connection.

• author can specify a custom cert for TLS
• not compatible with unix sockets (see: [flagd-RPC] Unix socket support #12)

[aepfli]

with the newest addition to our flagd-testbed (submodule test-harness in here) - there is now also a ssl image with a custom cert. which you can use