So far we've only handled legacy payloads, which meant we could drop the realm
byte since it was always 0x00. Once we start handling TLV payloads the first
byte, i.e., the former realm byte, is important since it gives us the length
of the payload. This is a breaking change, however I don't think there's
anyone using the `raw_payload` as of yet.
Changelog-Changed: JSON-RPC: the `raw_payload` now includes the first byte, i.e., the realm byte, of the payload as well. This allows correct decoding of a TLV payload in the plugins.