catalog: Add mz_overridden_system_parameters relation

[antiguru]

Adds mz_internal.mz_overridden_system_parameters, a materialized view exposing environment-wide system parameter overrides set via ALTER SYSTEM.
It mirrors the existing mz_cluster_system_parameters and mz_replica_system_parameters views, projecting the durable system_configurations collection out of mz_catalog_raw where the record kind is ServerConfiguration.

Today the only way to observe environment-wide system parameters is SHOW ALL, which surfaces a limited set unless the session is the mz_system user.
That user is too privileged for merely inspecting effective parameter values.
This view is PUBLIC_SELECT, so any user can read the overrides.

Like the cluster- and replica-scoped views, it lists only parameters with an explicit environment-wide override.
Parameters left at their default are absent.
Surfacing the effective value of every parameter, including unset defaults, would require reading live SystemVars rather than the durable overrides, and is left as a follow-up.

Tests: extends the catalog testdrive listing and the autogenerated mz_internal, oid, information_schema_tables, and catalog_server_explain sqllogictest snapshots.
The catalog_server_explain snapshot is made hermetic with a reset-server directive so role OIDs from preceding files no longer leak into its constant-folded plans.

This release will add the mz_overridden_system_parameters system catalog relation, listing environment-wide system parameter overrides.

🤖 Generated with Claude Code

[SangJunBak]

All in all, I really like this change and it'll make it much easier to debug self managed. The policy change of having system variables public to everyone seems okay to me, especially since users can find the default values from the repo and we're not storing secrets inside our system parameters. Code looks good to me too but I'll delegate the policy decision to @aljoscha

[mtabebe]

Minor comments. Good idea

[antiguru]

The policy change of having system variables public to everyone seems okay to me, especially since users can find the default values from the repo and we're not storing secrets inside our system parameters.

They could also just read from mz_internal.mz_catalog_raw. The information is there already, just so in a more digestible form.

Thanks for the reviews!

[SangJunBak]

They could also just read from mz_internal.mz_catalog_raw. The information is there already, just so in a more digestible form.

@antiguru Ah not exactly. That relation by default only gives access to the system user too. I still think it's okay to surface this to everyone, but just wanted to clarify!

Edit: Just tested it out and we actually allow superusers to query it too. I got confused by this comment https://github.com/SangJunBak/materialize//blob/main/src/catalog/src/builtin/mz_internal.rs#L48