[signer] RejectReason should be combined with RejectCode with proper backwards compatibility implementation

[jferrant]

Unfortunately in order to enable expanding RejectCode values without breaking backwards compatibility, #5827 added RejectReason to the BlockResponseData struct.

As part of the next hardfork, remove this add a proper RejectCode::Unknown(u8) which will enable proper backwards compatibilty and move all the RejectReasons to RejectCode.