rpc: Validate maxfeerate with AmountFromValue

Summary:

With this change maxfeerate can also be set as a string, accordingly to the help test:

maxfeerate (numeric or string,

Backport of Core PR15770

Test Plan: ninja all check-all

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7768