Add purpose arg to Wallet::getAddress

Summary:
Also make all arguments to getAddress required and document args at call sites.

This is a partial backport of Core PR12830 : https://github.com/bitcoin/bitcoin/pull/12830/commits/c5b277033a72650c221084ec0f1326623a810fd0

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3668