Rename DecodeDumpTime to ParseISO8601DateTime and move to time.cpp

Summary:

• Add roundtrip and more tests to ParseISO8601DateTime and FormatISO8601DateTime

This is a backport of Core PR17266

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5447