net: extend Sock with a method to check whether connected

Summary:
This will be convenient in the I2P SAM implementation.

This is a backport of core#20685 [13/20]
https://github.com/bitcoin/bitcoin/pull/20685/commits/5bac7e45e1d3a07115b5ff002d988438fcc92a53

Depends on D11023

Test Plan:
With clang-tidy,
ninja

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11024