feat(jwt): add dsl claims builder

[beatt83]

This feature will add the ability of creating JWTs claims with DSL.

Example:

let jwt = try JWT.signed(
        payload: {
            IssuerClaim(value: "testIssuer")
            SubjectClaim(value: "testSubject")
            ExpirationTimeClaim(value: Date())
            IssuedAtClaim(value: Date())
            NotBeforeClaim(value: Date())
            JWTIdentifierClaim(value: "ThisIdentifier")
            AudienceClaim(value: "testAud")
        },
        protectedHeader: DefaultJWSHeaderImpl(algorithm: .ES256),
        key: key
    ).jwtString