Add ArgsManager.GetDataDirBase() and ArgsManager.GetDataDirNet() as an intended replacement for ArgsManager.GetDataDirPath(net_identifier)

Summary:
This is a backport of core#21850 [2/6]
https://github.com/bitcoin/bitcoin/pull/21850/commits/0f53df47d5b8319fc71f3b87c15a115ff797fb50

Depends on D11952

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11953