Move RecoverDatabaseFile and RecoverKeysOnlyFilter into salvage.{cpp/h}

Summary:
Partial backport (9/11) of core PR18918:
https://github.com/bitcoin/bitcoin/pull/18918/commits/9ea2d258b46e8a9776100633585ed0feede5c2a4

Depends on D8595.

The DBKeys declaration are added to the walletdb.h file to make them
available for salvage.cpp, see D7020.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8596