feat: add `mount` component

Used to support GitLab CI runners that provide a persistent mount between jobs.

This can vastly speed up Bazel execution.

Bazel 8.3+ can use `--repo_contents_cache` to concurrently share the cached contents.

Set `CI_PERSISTENT_CACHE_DIR` in the GitLab CI variables to use. This is best set in the UI as it can change temporarily and should not be baked into the `.gitlab-ci.yml`.