sync: add --max-concurrent-requests flag

[igrekun]

Changes

Expose the previously-hardcoded request concurrency cap as a flag on the databricks sync command. Defaults to the existing limit (20) so behavior is unchanged when the flag is omitted; values < 1 are rejected.

Why

When uploading multiple files ~5mb size high concurrency can trigger stream timeouts

Tests

[github-actions]

Approval status: pending

/cmd/sync/ - needs approval

Files: cmd/sync/sync.go, cmd/sync/sync_test.go
Suggested: @simonfaltum
Also eligible: @renaudhartert-db, @hectorcast-db, @parthban-db, @tanmay-db, @Divyansh-db, @tejaskochar-db, @mihaimitrea-db, @chrisst, @rauchy

/libs/sync/ - needs approval

Files: libs/sync/sync.go, libs/sync/watchdog.go
Suggested: @simonfaltum
Also eligible: @renaudhartert-db, @hectorcast-db, @parthban-db, @tanmay-db, @Divyansh-db, @tejaskochar-db, @mihaimitrea-db, @chrisst, @rauchy

General files (require maintainer)

Files: acceptance/cmd/sync-without-args/output.txt
Based on git history:

• @pietern -- recent work in libs/sync/, cmd/sync/, acceptance/cmd/sync-without-args/

_{Any maintainer (@andrewnester, @anton-107, @denik, @pietern, @shreyas-goenka, @simonfaltum, @renaudhartert-db) can approve all areas.
See OWNERS for ownership rules.}