Add --oidc-provider flag to specify which provider to use for ambient credentials

[priyawadhwa]

This should help fix #1993

This way we can use the flag to specify one provider (e.g. spiffe) while still having credentials for another mounted in (e.g. google)

Add --oidc-provider flag to specify which provider to use for ambient credentials

cc @strongjz

[codecov-commenter]

Codecov Report

Merging #1998 (e1c56fd) into main (b01a173) will decrease coverage by 2.52%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #1998      +/-   ##
==========================================
- Coverage   28.69%   26.16%   -2.53%     
==========================================
  Files         133      127       -6     
  Lines        8092     7422     -670     
==========================================
- Hits         2322     1942     -380     
+ Misses       5463     5225     -238     
+ Partials      307      255      -52     

Impacted Files	Coverage Δ
cmd/cosign/cli/attest.go	0.00% <0.00%> (ø)
cmd/cosign/cli/fulcio/fulcio.go	18.10% <0.00%> (-1.53%)	⬇️
cmd/cosign/cli/options/oidc.go	0.00% <0.00%> (ø)
cmd/cosign/cli/policy_init.go	1.39% <0.00%> (-0.01%)	⬇️
cmd/cosign/cli/sign.go	0.00% <0.00%> (ø)
pkg/cosign/tuf/status_type.go
pkg/cosign/tuf/signer.go
pkg/cosign/tuf/client.go
pkg/cosign/tuf/policy.go
pkg/cosign/tuf/usage_type.go
... and 3 more

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[dlorenc]

Missing DCO, otherwise LGTM!

[priyawadhwa]

Whoops, fixed!