Add system variables for config.yaml settings, like `archive_level` and `auto_gc`

Dolt v1.53.2

Currently, some server features must be set in `config.yaml`, but there is no corresponding system variable to read against the live server to inspect whether these features have been enabled.

Two settings in particular would be nice to have as system variables:
`auto_gc_enabled`
`archive_level`

I think Postgres has a variable for its automatic garbage collection, like:
```sql
SHOW autovacuum;
-- returns "on" or "off"
```

Start by tracing how settings from config.yaml are loaded and how existing SQL system variables expose live server state. Compare the requested auto_gc_enabled and archive_level behavior with the referenced SHOW autovacuum example. Done means both settings can be queried as system variables and accurately reflect the running server configuration.

Written by the indexing model from the issue text.