configure server-side TLS on grpc listener

[bobcallaway]

Summary

In some deployment scenarios, it may be useful to allow a fulcio instance to listen for gRPC over TLS without the presence of a fronting load balancer. This adds two new command line arguments which allow providing the private key and certificate that can be used to support enabling TLS on the grpc port

Release Note

• grpc-port can now listen with TLS support using --grpc-tls-certificate and --grpc-tls-key command line arguments

[codecov]

Codecov Report

Merging #1252 (1985bd7) into main (8d1b285) will increase coverage by 0.10%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##             main    #1252      +/-   ##
==========================================
+ Coverage   56.13%   56.24%   +0.10%     
==========================================
  Files          50       50              
  Lines        2900     2921      +21     
==========================================
+ Hits         1628     1643      +15     
- Misses       1129     1134       +5     
- Partials      143      144       +1     

Impacted Files	Coverage Δ
cmd/app/grpc.go	45.94% <78.57%> (+4.64%)	⬆️
cmd/app/serve.go	27.24% <100.00%> (+0.50%)	⬆️