Allow store logger to be configured and update some references to slog default logger

[hfoxy]

Description

Updates the usages of slog.Info|Warn|Error|Debug to use either the currently defined logger on litestream.DB or a newly defined logger on litestream.Store

Motivation and Context

Fixes #1178

How Has This Been Tested?

Ran tests and have deployed this commit to a testing environment with ~27 databases constantly being updated.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (would cause existing functionality to not work as expected)

Checklist

• My code follows the code style of this project (go fmt, go vet)
• I have tested my changes (go test ./...)
• I have updated the documentation accordingly (if needed)

[corylanou]

Hey! Thanks for this PR. Just a heads up — #1181 was merged and covers all the store.go changes (adding Store.Logger and replacing slog.* calls there).

After a rebase onto main, the remaining diff from this PR should be the 3 db.go changes in verify() (slog.Debug → db.Logger.Debug), which are still valuable and not yet addressed. Would be great to rebase and keep those!