Add flag for parquet queries concurrency in parquet bucket store

[SungJin1212]

This PR introduces a new configuration flag for setting the concurrency of parquet queries in the parquet bucket store.
Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]
• docs/configuration/v1-guarantees.md updated if this PR introduces experimental flags

[friedrichg]

The flag gates three nested errgroups, where each level holds its slot while waiting on the level below, so the per-request concurrency compounds: effective concurrency is value^3, not value. With the default 4, a single request fans out to up to 64 goroutines (4 × 4 × 4). With 16 it's 4096.

The three levels:

1. Series over shards (parquet_bucket_store.go:113)
2. parquetBlock.Query over row groups (parquet_bucket_stores.go:388)
3. search.NewMaterializer over columns (parquet_bucket_stores.go:330)

This applies per request (no shared limiter even within a tenant), so total goroutines = value^3 × in-flight requests.

Either we change the flag help text, or we split into three flags so each level can be tuned independently.

[yeya24]

It is one concurrency flag gating all concurrency in multiple places. Same in the upstream library. We can document that it is not 4 goroutines for the whole query just to be clear. To tune this independently this requires upstream change and introduces more complexity