Add pf_invalid arg to std::string DecodeBase{32,64}
Summary:
Add support for the optional "pf_invalid" out parameter (which allows the caller to detect decoding failures) to the std::string versions of DecodeBase32 and DecodeBase64. The char* versions already have this feature. Also, rename all uses of pfInvalid to pf_invalid to match style guidelines.
Partial backport of core PR14978:
https://github.com/bitcoin/bitcoin/pull/14978/commits/162ffefd2f562169725559906601c25c579aa91c
Depends on D5735.
Test Plan:
ninja all check check-functional
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D5736