Use Issuer interface to allow for custom issuers

[priyawadhwa]

This plumbs through IssuerPool and starts using it to set up Fulcio.

[haydentherapper]

LGTM with the concept, as we chatted about, server implementers who want to use private implementations of an issuer will instantiate the gRPC server with their own issuer pool.

[haydentherapper]

Will take a look Monday!

[priyawadhwa]

@haydentherapper just so you know it's still a WIP, have a couple more things to add!

[codecov]

Codecov Report

Merging #1008 (1bc60cd) into main (d15bdf8) will increase coverage by 0.30%.
The diff coverage is 90.66%.

@@            Coverage Diff             @@
##             main    #1008      +/-   ##
==========================================
+ Coverage   54.47%   54.77%   +0.30%     
==========================================
  Files          46       48       +2     
  Lines        2480     2521      +41     
==========================================
+ Hits         1351     1381      +30     
- Misses       1030     1039       +9     
- Partials       99      101       +2     

Impacted Files	Coverage Δ
cmd/app/serve.go	26.73% <40.00%> (-0.19%)	⬇️
pkg/identity/base/issuer.go	88.46% <88.46%> (ø)
pkg/server/issuer_pool.go	96.66% <96.66%> (ø)
cmd/app/grpc.go	41.30% <100.00%> (ø)
pkg/identity/buildkite/issuer.go	62.50% <100.00%> (-7.50%)	⬇️
pkg/identity/email/issuer.go	62.50% <100.00%> (-7.50%)	⬇️
pkg/identity/github/issuer.go	62.50% <100.00%> (-7.50%)	⬇️
pkg/identity/kubernetes/issuer.go	62.50% <100.00%> (-7.50%)	⬇️
pkg/identity/spiffe/issuer.go	62.50% <100.00%> (-7.50%)	⬇️
pkg/identity/uri/issuer.go	62.50% <100.00%> (-7.50%)	⬇️
... and 5 more

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[priyawadhwa]

@haydentherapper this should be RFAL! Thanks :)

[haydentherapper]

Do you want to delete https://github.com/sigstore/fulcio/blob/main/pkg/challenges/challenges.go#L53-L83 also?

[priyawadhwa]

@haydentherapper i was planning on deleting everything in a follow-up! wanted to try pulling in this commit and make sure everything worked before deleting anything.

[haydentherapper]

Sounds good! Thanks!