Add a GetMinimumFeeRate function which is wrapped by GetMinimumFee

Summary: This is a partial backport of Core PR10637 : https://github.com/bitcoin/bitcoin/pull/10637/commits/fab04887c22cf71a4b15ac0b0a127e150229d686

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3251