Add wallet backup text to import* and add* RPCs

Summary:
Add wallet backup text to import*, add* and dumpwallet RPCs

Wrap dumpwallet warning and note scripts aren't dumped

This is a backport of Core PR11289

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1919