rpc: Add getblockfilter RPC method.

Summary:
Retrieves and returns block filter and header from index.

This is a partial backport of Core PR14121 : https://github.com/bitcoin/bitcoin/pull/14121/commits/19308c9e21d98a7818625218b22b37f23f87816f

Depends on D6346

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6349