[rpc] return effective-feerate in testmempoolaccept and submitpackage

Summary:
Partial backport of core#26646:
https://github.com/bitcoin/bitcoin/pull/26646/commits/1691eaa818f7a7b22907f756490b842d80a9a21d

Depends on D16397.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, roqqit

Reviewed By: roqqit

Differential Revision: https://reviews.bitcoinabc.org/D16398