* feat: add signal API support

Add support for signal API

* feat: add signal listener method to protocol

Add new callback to 'PNObjectEventListener', so existing listeners may start receiving 'signal'
objects by adding new callback implementation.

perf: disable pipelining

Disable pipelining for requests, because there is no guarantee, what responses will arrive in 
same order as they has been requested - this may lead to unexpected response in used API. There 
is a lot of factors which is out of user control and they affect proper pipelining functionality.

* chore: disable encryption / decryption for signals

Remove signal payload encryption and decryption from subscribe response parser.

refactor: remove 'stripMobilePayload' completely from SDK

Remove deprecated configuration option from SDK along with code, which used it.

* fix: decryption error message de-duplication crash

Fixed crash which is caused by attempt to de-duplicate message which PubNub client wasn't able to
decrypt with configured 'cipherKey'

test: enhance unencrypted signal receive test

Add sub-test which ensure, what client will fail to decrypt regular message and won't notify same
kind of error for signal which is not encrypted.