Add serde for Signature types (#4109)

* refactor: Added `from_slice()` method to ECDSA signatures

* doc: Modified ECDSA signature docstring to note Recovery ID

* feat: Implemented serde for Signature types

Note: using hexstring encoding

* feat: Automatically derive serde for MultiSignature

* refactor: Convert hex bytes using try_from instead of from_slice

Avoids panicking in critical code

Implemented from Peer Review

* clean: spaces -> tabs

* test: Added tests for Signature serialization

Added dependency on serde_json for testing purposes