persist: skip lgalloc memcpy in S3 get path

[antiguru]

The S3 get path copied each Bytes chunk returned by the SDK into a freshly-allocated MetricsRegion<u8>.
With lgalloc disabled in practice that region falls back to the heap, making the copy pure overhead.
Push SDK Bytes straight into SegmentedBytes instead.

Also removes the now-unused persist_enable_s3_lgalloc_{cc,noncc}_sizes dyncfgs, the cfg/is_cc_active fields on S3BlobConfig/S3Blob, and the dead LgBytesMetrics::persist_s3 metric.

Motivation

Reduces CPU and allocator pressure on every S3 read in persist.

Tips for reviewer

The behavior change is in Blob::get in src/persist/src/s3.rs; everything else is dead-code removal that follows.

Checklist

• This PR has adequate test coverage / QA is not needed.
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.

[antiguru]

We should do the same for other parts, too, like azure and arrow.

[DAlperin]

LGTM thanks!