[txorphanage] add method to get all orphans spending a tx

Summary:
And add some unit tests for the new functions.

Partial backport of core#28970:
https://github.com/bitcoin/bitcoin/pull/28970/commits/2f51cd680fb4323f1c792dae37d4c4e0e0e35804
https://github.com/bitcoin/bitcoin/pull/28970/commits/d095316c1c23e9460dfbd9fdbaf292063adcd080

Depends on D16491.

Test Plan:

ninja check-bitcoin-orphanage_tests

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16493