Merge #13399: rpc: Add submitheader

Summary:
fa091b001605c4481fb4eca415929a98d3478549 qa: Add tests for submitheader (MarcoFalke)
36b1b63f20cc718084971d2cadd04497a9b72634 rpc: Expose ProcessNewBlockHeaders (MarcoFalke)

Pull request description:

This exposes `ProcessNewBlockHeaders` as an rpc called `submitheader`. This can be used to check for invalid block headers and submission of valid block headers via the rpc.

Tree-SHA512: a61e850470f15465f88e450609116df0a98d5d9afadf36b2033d820933d8b6a4012f9f2b3246319c08a0e511bef517f5d808cd0f44ffca91d10895a938004f0b

Backport of Core PR13399
https://github.com/bitcoin/bitcoin/pull/13399/files
Completes T685
Depends on D3856

Test Plan:

make check
test_runner.py

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4046