rpc: Register calls where they are defined

Split out methods to every module, apart from 'help' and 'stop' which
are implemented in rpcserver.cpp itself.

- This makes it easier to add or remove RPC commands - no longer everything that includes
    rpcserver.h has to be rebuilt when there's a change there.
- Cleans up `rpc/server.h` by getting rid of the huge cluttered list of function definitions.
- Removes most of the bitcoin-specific code from rpcserver.cpp and .h.

Continues #7307 for the non-wallet.

src/Makefile.am

@@ -130,6 +130,7 @@ BITCOIN_CORE_H = \
   rpc/client.h \
   rpc/protocol.h \
   rpc/server.h \
+  rpc/register.h \
   scheduler.h \
   script/sigcache.h \
   script/sign.h \