Add a new entity child table for storing Wallet metadata.

This change adds a table autofill_ai_entities_metadata_wallet for
storing metadata that is specific for Wallet-managed entities.
For now, this only comprises a management URL for deeplinking
into Wallet.

The change also adds DB migration code and logic in the
database layer for reading and writing the new table.

Bug: b:560061580
Change-Id: Ia9cf18c65a3f776ca5d9cf19fea745766a6a6964
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8428007
Reviewed-by: Norge Vizcay <vizcay@google.com>
Commit-Queue: Thomas Schickinger <schickin@google.com>
Reviewed-by: Florian Leimgruber <fleimgruber@google.com>
Cr-Commit-Position: refs/heads/main@{#1706770}