make `br debug checksum` multi-threaded

## Enhancement

Currently `br debug checksum` runs the checksum comparison in a single thread. For a huge archive with many SST files this may not be able to utilize all resources.

https://github.com/pingcap/tidb/blob/95181316d58ba47092567c115fb5734e02a19ef0/br/cmd/br/debug.go#L90-L96

There should be a `--concurrency` flag to run multiple hashsum checks in parallel.

Start at br/cmd/br/debug.go around lines 90-96 and trace how `br debug checksum` performs its checksum comparison. Check the command's existing flag setup and identify how the SST files are enumerated before deciding where concurrency belongs. Done means the command accepts `--concurrency` and runs multiple hashsum checks in parallel for large archives.

Written by the indexing model from the issue text.