feature: Added ability for users to add a startup command

Summary:

Thoughts for adding the feature is for users to be able to add things like electrum-personal-server or lnd to run whenever Bitcoin Core is running. Open to feedback about the feature.

This is a backport of core#15367

Test Plan: ninja && src/bitcoind -startupnotify="notify-send 'bitcoind started' 'test message'"

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10346