* Add periodic import of database objects from supported databases.
A single implementation is included: for Postgres, along with necessary changes.
Also refactors SQLAdminClient interface to accept cert expiry instead of pulling that information from `tlsca.Identity`.
* fix logging, imports
* add missing license
* review feedback
- several minor changes
- apply import rules for all registered object fetchers using wrapper type
- refactor DAC to use object fetcher
- adjust tests
* add comment and missing log call
* support config overrides through unstable env vars, including disabling the service entirely
* refactoring and minor changes
- be more specific about required API for ObjectFetcherConfig
- measure scan time
- bump default scan interval
- call StopImporter from stopDatabase to fix issue with dynamic db resource updates
- add comments
* Refactor `connectAsAdmin`, add `withDefaultDatabase()` method for clarity of implementation.
* Reduce surface area for fetcher config
* make the not-implemented case less noisy by skipping the error
* lint fix
* decrease verbosity of reporting for unsupported configurations
* make `reason` field private
* move `unregisterObjectFetcher` to tests
* avoid exceedingly long lines with log calls
* correct formatting leftover from slog conversion
* tighten logic around disabled importer
if the scan interval is zero or less, the importer becomes disabled, which avoids panic from interval.New().
add some tests and comments for clarity.
* drop unused method
