WIP: split OIDC providers into separate Go modules

[imjasonh]

This PR demonstrates splitting the current single Go module into a number of smaller Go modules:

• ./pkg/providers/{buildkite,google,spiffe} implements a provider for various OIDC creds, which rely on sometimes-heavy helper libraries
• ./pkg/providers/all is a convenience to import the above modules; it's deprecated in this PR
• ./cmd/cosign is the CLI, which depends on the OIDC providers
• ./ is the root module, which is "cosign as a library", now without many heavy dependencies
• ./test depends on ./cmd/cosign and provides e2e tests
• ./cmd/help depends on ./cmd/cosign [WIP]

Because some functionality and tests in the root module need them:

• CA testing helpers are moved from ./test/ to ./pkg/cosign/test
• predicate types are moved from ./cmd/cosign/cli/options to ./pkg/cosign/policy

Previously, inspired by sigstore/sigstore#1115

TODO

• adding tags for sub-Go-modules, like we do in s/s
• make test to test each module, since go test ./... won't anymore
• e2e test breakages

[codecov]

Codecov Report

Merging #3059 (53b6a19) into main (51c9bc2) will increase coverage by 18.27%.
The diff coverage is 42.85%.

@@             Coverage Diff             @@
##             main    #3059       +/-   ##
===========================================
+ Coverage   30.64%   48.91%   +18.27%     
===========================================
  Files         155       68       -87     
  Lines        9791     4657     -5134     
===========================================
- Hits         3000     2278      -722     
+ Misses       6341     2077     -4264     
+ Partials      450      302      -148     

Impacted Files	Coverage Δ
pkg/policy/attestation.go	31.57% <42.85%> (ø)

... and 87 files with indirect coverage changes

[github-actions]

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[github-actions]

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