[radix] Adds the capability to use an adapter for key manipulation

Summary: This allow to use element which do not have a getId memeber, or do not have a valid key type in the tree.

Test Plan:

ninja check-bitcoin-radix_tests

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D11456