Create interface for GRPC server which encompasses the GRPC HealthServer

[priyawadhwa]

This PR updated the NewGRPCCAServer function to return a struct rather than an interface. I think this was done to make it easier to set up the GRPC health server.

We use this function downstream, and it's much more complicated for us to write tests (previously we could mock the NewGRPCCAServer function and return a mock interface).

This PR basically changes back to returning an interface, and creates a new GRPCCAServer which embeds the usual fulcio server and the grpc health check server.

[codecov]

Codecov Report

Merging #1334 (3976a05) into main (d740733) will decrease coverage by 0.10%.
Report is 1 commits behind head on main.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##             main    #1334      +/-   ##
==========================================
- Coverage   58.59%   58.49%   -0.10%     
==========================================
  Files          50       50              
  Lines        3053     3053              
==========================================
- Hits         1789     1786       -3     
- Misses       1108     1110       +2     
- Partials      156      157       +1     

Files Changed	Coverage Δ
pkg/server/grpc_server.go	50.55% <71.42%> (ø)

... and 1 file with indirect coverage changes