JWT library with flexible signers

[kdimak]

We use https://github.com/square/go-jose to deal with JWT in verifiable package.
In order to sign the JWT, go-jose requires creation of Signer object with the mandatory passing of a private key. We would like to deal with private keys inside KMS solely and thus this approach does not satisfy us.

Requirements for the light-weight impl of the JWT:

• Signer interface does not require private key.
• Support JWS and unsecured JWT.
• Support detached payloads (to be used by signature package.
• Support b64 header.
• Support RS256 and EdDSA verifications from the box (as they are used in verifiable).
• To be opened for extra algorithms (e.g. secp256k1).

[kdimak]

Follow-up issues:

• Use crypto in jwt package for signing/verification logic (Use crypto in "jwt" package for signing/verification logic #1278)
• JWT: ECDSA to support NIST P256 curve (JWT: ECDSA to support NIST P256 curve. #1266)
• Pluggable JWS verifiers (Pluggable JWS verifiers #1267)
• Support JWS JSON serialization format (support JWS JSON serialization format #1331)