[roll] Roll fuchsia [bazel2gn] Implement batch mode in bazel2gn

Adds support for syncing multiple directories in parallel using a worker
pool in Go. Directories to sync can be specified via the
`--directory_list` flag or as positional arguments.

Updates tools/devshell/contrib/bazel2gn to use the new batch mode.

This significantly improves the runtime of `fx bazel2gn`, and it will
scale much better as we add more files to sync.

Benchmarking out/default/bazel2gn_dir_list with 626 directories to sync,
runtime of the bazel2gn binary in `fx bazel2gn` went from 6.5s to 0.3s.
Overall runtime of `fx bazel2gn`, including `fx gen` and
`bazel build bazel2gn` went from 27s to 20s.

NOTE: Batch mode is not enabled for //build:bazel2gn_verifications yet,
which runs all bazel2gn verifications during the build. Ninja will
parallelize the execution of the large number of bazel2gn anyways.

Original-Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1623676
Original-Original-Revision: 74652f248e6204e0c0c6bd84778d0c47927b4296
GitOrigin-RevId: 2dee9c67cd71badbd62b77292bdab665abbc8598
Change-Id: Ib32c0c24db956e5102833c95017d76895b9fede3
Reviewed-on: https://fuchsia-review.googlesource.com/c/integration/+/1638814
Cr-Commit-Position: refs/heads/main@{#194058}