Add keys to source package

Summary:
contrib/gitian-signing contains the fingerprints of the release signing keys as well as a script to check/refresh those keys.
These are useful to users wishing to verify the integrity of their source and binaries.

Inspired by https://reviews.bitcoinabc.org/D4807#116075

Test Plan:

ninja package_source
tar -xzvf bitcoin-abc-0.20.10.tar.gz
ls bitcoin-abc-0.20.10/contrib/gitian-signing  # verify files are part of the source package

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D4828