ArgsMan: Add Get*Arg functions returning optional

Summary:
This allows the caller to not provide a default at all and just check
inside the optional to see if the arg was set or not.

This is a partial backport of core#25290
https://github.com/bitcoin/bitcoin/pull/25290/commits/fc02f77ca604f0221171bfde3059b34f5d0fb1cd

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15297