Move the declaration of various json conversion function in rpc/tojson.h

Summary: They are delcared in multiple places, which is not very maintainable.

Test Plan:

make check
../qa/pull-tester/rpc-tests.py

Reviewers: schancel, #bitcoin_abc

Reviewed By: schancel, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D717