add HTTP and GRPC health check endpoints

[bobcallaway]

Summary

This adds both HTTP and gRPC health check endpoints to Fulcio.

HTTP: /healthz should return HTTP 200 OK
gRPC: implements gRPC Health Checking Protocol

This also adds coverage for this via K8S local testing using readiness and liveness probes in cluster.

Release Note

• HTTP and gRPC health check endpoints are now exposed from Fulcio

Documentation

• update docs.sigstore.dev to denote this feature

[codecov]

Codecov Report

Merging #1258 (827ebb9) into main (a5b774d) will increase coverage by 0.18%.
The diff coverage is 57.69%.

@@            Coverage Diff             @@
##             main    #1258      +/-   ##
==========================================
+ Coverage   56.07%   56.25%   +0.18%     
==========================================
  Files          50       50              
  Lines        2921     2940      +19     
==========================================
+ Hits         1638     1654      +16     
- Misses       1138     1140       +2     
- Partials      145      146       +1     

Impacted Files	Coverage Δ
cmd/app/http.go	51.35% <46.15%> (-2.62%)	⬇️
pkg/server/grpc_server.go	50.55% <55.55%> (-1.15%)	⬇️
cmd/app/grpc.go	47.82% <100.00%> (+1.88%)	⬆️

... and 2 files with indirect coverage changes

[haydentherapper]

Any tests for this new endpoint? Would be ideal to see both HTTP and gRPC.

[bobcallaway]

Any tests for this new endpoint? Would be ideal to see both HTTP and gRPC.

The addition in deployment.yaml gets us coverage on both as part of the k8s tests.