Allow decoding of `[String]` JWK parameters

Currently, we ignore (via #116) the JWK parameters X509CertificateChain and keyOperations because their type is [String]. This is because we only decode into a [String: String] dictionary at the moment. We should enable the decoding of String and [String] to be standard compliant. The relevant code is in RSAKeysCodable.swift and in the JWK types themselves e.g. in RSAPublicKey.swift.

Possible parameters and their types are listed in RFC-7517:

• "kty": String
• "use": String
• "key_ops": [String]
• "alg": String
• "kid": String
• "x5u": URI
• "x5c": [String]
• "x5t": String
• "x5t#S256": String