Deduplicate some block-to-JSON code.

Summary:
Some of the logic converting blocks and block headers to JSON for
the blockchain RPC methods (getblock, getlockheader) was duplicated.
Instead of that, the blockToJSON RPC method now calls blockheaderToJSON
first, and then fills in the missing, block-specific bits explicitly.

This is a backport of core#21114

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11335