feat(mTLS): adds mTLS support to dataplane api-server

[EandrewJones]

Closes #50.

Questions

I am not sure how best to add tests for this. Unit testing the config trivially tests the behavior of Opt which we know works. Unit testing setup_tls forces me to mock read_to_string in the absence of an actual file to read in and all the ways I've read about doing that in rust feel really kludgie / dirty up the code with dependency injections, etc.

1. Is this all we need to add mTLS support for the api-server?
2. Is there some sort of API contract we need to maintain for this such that I can write a conformance or integration test?
3. What documentation would we like to see for this?