secure the gRPC server with TLS

[orbitalturtle]

In a nutshell this PR:

• Requires TLS to be used when connecting to the gRPC server, and generates self-signed certificates for doing so.
• Updates lndk-cli to meet this requirement.
• Updates the documentation.

Note that the cert generation portion takes a little inspiration from TEOS and CLN. Credit where it's due!

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 21 lines in your changes missing coverage. Please review.

Project coverage is 0.00%. Comparing base (b341ace) to head (15bbe3f). Report is 1 commits behind head on master.

Files	Patch %	Lines
src/main.rs	0.00%	21 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #112 +/- ## ====================================== Coverage 0.00% 0.00% ====================================== Files 1 1 Lines 77 96 +19 ====================================== - Misses 77 96 +19 ```

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