Keystore 2.0: Add metadata table to KeystoreDb.

* Add a macro that generates infrastructure for key metadata fields
  that can be associated with a key without having to modify the table
  topology.
* Moved SqlField to db_utils.rs.

Test: keystore2_test
Bug: 173545997
Change-Id: Ic30186ec93679700db404180a69775c19d40e8e2
3 files changed