feat: Support loading and managing keys from files

[rdimitrov]

Moved from rdimitrov/go-tuf-metadata#21, probably will be obsolete once the signer/verifier consolidation effort gets traction.

API to:

• load/unload private/public PEM formatted keys from file
• to/from crypto.PublicKey/crypto.PrivateKey
• from there on one can choose to further translate it to TUF Key type

References:

• https://github.com/sigstore/sigstore/blob/e2c1f8c3ccaf13f270430895a0142e58635eefad/pkg/cryptoutils/publickey.go
• https://github.com/sigstore/sigstore/blob/e2c1f8c3ccaf13f270430895a0142e58635eefad/pkg/cryptoutils/privatekey.go