Condense key format conversion helpers

[Moopli]

We have code scattered around to convert keys between different formats/representations:

• DID Verification Method
• JWK
• Key bytes (sometimes in multiple different formats between uses) + kms.KeyType
• standard library crypto keys (ecdsa.PrivateKey, etc)
• Tink key handles & KMS Key IDs

I'm thinking we should make JWK the primary representation (outside of KMS/Crypto), since we already have support for converting to/from JWK for stdlib crypto keys, and bytes+kms.KeyType. For Tink key handles & KMS Key IDs, the KMS api supports converting to/from bytes+kms.KeyType - helper functions could consolidate to pass/receive JWK directly, and potentially update KMS API in future. DID Verification Methods either hold a JWK, or encoded key bytes with a type identifier (different from kms.KeyType), so VerificationMethod could get an API upgrade to support returning non-JWK keys as JWKs.