move utility key handling functions in pkg/cosign to an internal package

[dmitris]

Description
Follow-up on the discussions in #3776 - we need to review the functions in https://github.com/sigstore/cosign/blob/main/pkg/cosign/keys.go and move some of them to an internal package (possibly on a major version update for the API backward compatibility; can mark functions as Deprecated until then).

As for the TODOs, I agree this should be in an internal package, this isn't meant to be general purpose.
- @haydentherapper

It may be helpful to view the old TODO(jason) comments in pkg/cosign/keys.go@bdcbf44ee9.

[dmitris]

files that use some of the types and functions in pkg/cosign/keys.go marked with TODO - move to internal, to help
evaluating the impact of the possible changes:

$ rg -l 'cosign\.KeysBytes'
test/helpers.go
pkg/cosign/kubernetes/secret_test.go
pkg/cosign/kubernetes/secret.go
cmd/cosign/cli/generate/generate_key_pair.go

$ rg -l 'cosign\.GeneratePrivateKey'
internal/pkg/cosign/ephemeral/signer.go
internal/pkg/cosign/payload/signer_test.go
internal/pkg/cosign/tsa/signer_test.go
internal/pkg/cosign/fulcio/signer_test.go
internal/pkg/cosign/rekor/signer_test.go
cmd/cosign/cli/sign/sign.go
cmd/cosign/cli/fulcio/fulcio_test.go

$ rg -l 'cosign\.ImportKeyPair'
test/helpers.go
cmd/cosign/cli/importkeypair/import_key_pair.go

$ rg -l 'cosign\.GenerateKeyPair'
test/helpers.go
cmd/cosign/cli/attest/attest_blob_test.go
cmd/cosign/cli/publickey/public_key_test.go
pkg/cosign/git/github/github.go
pkg/cosign/git/gitlab/gitlab.go
pkg/cosign/kubernetes/secret.go
test/e2e_test.go
cmd/cosign/cli/generate/generate_key_pair.go

# NB PemToECDSAKey is used only in the pkg/cosign package, pkg/cosign/keys.go file (no cosign.PemToECDSAKey references):
$ rg PemToECDSAKey
pkg/cosign/keys.go
197:// PemToECDSAKey marshals and returns the PEM-encoded ECDSA public key.
198:func PemToECDSAKey(pemBytes []byte) (*ecdsa.PublicKey, error) {

$ rg -l 'cosign\.LoadPrivateKey'
test/e2e_test.go