refactor: introduce single-separator split helper SplitString
Summary:
This helper uses spanparsing::Split internally and enables to replace all calls to boost::split where only a single separator is passed.
Backport of core#22953.
Test Plan:
ninja all check-all ninja bitcoin-fuzzers
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D14091