Make DecodeBase{32,64} take string_view arguments

Summary:

Generalize ConvertBits to permit transforming the input

https://github.com/bitcoin/bitcoin/pull/25001/commits/1a72d62152bfdd7c5c2b2704b679f894e7d35e37

Make DecodeBase{32,64} take string_view arguments

https://github.com/bitcoin/bitcoin/pull/25001/commits/8ffbd1412d887535ce5eb613884858c319bd12be

This is a partial backport of core#25001
Depends on D17149

Test Plan: ninja all check-all bitcoin-fuzzers

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17150