Claims JWT Support

[arnavdas88]

Added Pythonic Interface for creating Account Claims and User claims as JWT.

Issue: Issue 717

Replicated the example at NATS by Example - Programmatic NKeys and JWTs in the test file tests/contrib/test_jwt.py.

Example Code Snippet

operatorKP = from_seed(b'SOALU7LPGJK2BDF7IHD7UZT6ZM23UMKYLGJLNN35QJSUI5BNR4DJRFH4R4')
accountKP = from_seed(b'SAALXUEDN2QR5KZDDSH5S4RIWAZDM7CVDG5HNJI2HS5LBVYFTLAQCOXZAU')
userKP = from_seed(b'SUALJTG5JNRQCQKFE652DV4XID522ALOHJNQVHKKDJNVGWHCLHOEXEROEM')

account_claims = Claims(
    name="my-account",
    jti="PBFES33GGIFZM6UGC7NY5ARHRBFVFU4UD7FS2WNLZH3KPGWFVEFQ",
    iat=1678973945,
    iss=operatorKP.public_key.decode(),
    sub=accountKP.public_key.decode(),
    nats=Account(
        limits=OperatorLimits(
            nats_limits=NatsLimits(data = -1, payload = -1, subs = -1),
            account_limits=AccountLimits(exports = -1, imports = -1, wildcards = True, conn = -1, leaf = -1),
            jetstream_limits=JetStreamLimits(disk_storage=-1, mem_storage=-1)
        ),
        default_permissions=Permissions(),
        generic_fields=GenericFields(version=2, type=Types.Account)
    )
)

account_claims.to_dict()

Some issues with the PR

• Directory and File structure is a mess in the contrib/.
• Test for Operator Claim is not written yet as the operator claims are not clear in NATS by Example - Programmatic NKeys and JWTs blog.
• encode_user_claim() and encode_account_claim() is not yet mature and only exists in the test file.
• The tests pass with the example shown in NATS by Example - Programmatic NKeys and JWTs, but the resultant jwt has not been tested with a nats-server.
• Adding code documentation can be helpful.

Note : Directory and File structure is a mess in the contrib/ because following any other file structure was resulting in circular import error, and putting all the code in a single file was making it hard to read. Any suggestions / commits on how to have a better structure for the files in contrib/ directory are welcome!

[arnavdas88]

Some (foreign and unrelated to this PR) tests in test_js.py are failing. Can someone confirm this behaviour?

@caspervonb @GarraPeters @wallyqs