Feature/1305 symmetric decryption

[maratal]

Partially closes ably/docs#1305

1. I've only added decryption for ARTMessage, because of absence of encrypted test data for ARTPresenceMessage. I could use the same file but I'm not sure, lets discuss.
2. fromEncodedJsonArray omits error while iterating and calling decodeWithEncoder for each item, because in the original code (from ARTRestChannel.history) this error goes nowhere except logger. And in this static function I have no logger, except one I can directly pass into fromEncodedJsonArray, but that would be in contradiction with spec requirement for this function signature (TM3).

[lawrence-forooghian]

@maratal, I don't think we can say "Closes #1305" if we're not handling PresenceMessage here. We should either handle it here or split that issue in two.

[maratal]

I don't think we can say "Closes ably/docs#1305" if we're not handling PresenceMessage here. We should either handle it here or split that issue in two.

Since there is no reliable way to detect which type of message in the provided dictionary, I have nothing to do except add similar methods to ARTPresenceMessage class. I'll do it once we agree upon all changes for ARTMessage. @lawrence-forooghian

[maratal]

Since there is no reliable way to detect which type of message in the provided dictionary, I have nothing to do except add similar methods to ARTPresenceMessage class. I'll do it once we agree upon all changes for ARTMessage.

I agree to create one more PR to actually fully address corresponding issue (I'll reopen it after this PR merged).