Extend validateaddress information for P2SH-embedded witness

Summary:
This adds new fields 'pubkeys' and 'embedded' to the RPC's output, and improves the
documentation for previously added 'witness_version' and 'witness_program' fields.

This is a partial backport of Core PR11403 : https://github.com/bitcoin/bitcoin/pull/11403/commits/3eaa003c888f80207e8ff132f78417ff373ddfa3

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Subscribers: markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D3129