feat(sql): add ALTER SYSTEM file cache clear

[Li0k]

Summary

This PR is stacked on #26577 and adds a SQL entry point for the runtime file-cache clear RPC:

ALTER SYSTEM CLEAR FILE CACHE META;
ALTER SYSTEM CLEAR FILE CACHE DATA;
ALTER SYSTEM CLEAR FILE CACHE ALL;

• Require superuser privileges in the Frontend handler.
• Add a Meta RPC that fans out to all ComputeNodes.
• Reuse the existing ResizeCacheRequest clear flags and runtime HybridCache::clear() implementation.
• Return an error if a ComputeNode cannot be cleared.

The first commit is the base implementation from #26577; the incremental SQL change is commit 3f42fc848a.

Validation

• cargo test -p risingwave_sqlparser parse_alter_system_clear_file_cache
• cargo check -p risingwave_meta_service -p risingwave_frontend -p risingwave_rpc_client -p risingwave_sqlparser
• cargo fmt --all
• git diff --check

[Li0k]

• feat(sql): add ALTER SYSTEM file cache clear #26578 👈 (View in Graphite)
• li0k/hummock-file-cache-clear

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