[notifier] Add section for pending deployments

[nezdolik]

Sample output:

nezdolik@lima-default1:/Users/nezdolik/Documents/code2/envoy/tools/repo$ bazel run --config=ci //tools/repo:notify -- --dry_run
INFO: Analyzed target //tools/repo:notify (0 packages loaded, 0 targets configured).
INFO: Found 1 target...
Target //tools/repo:notify up-to-date:
  bazel-bin/tools/repo/notify
INFO: Elapsed time: 0.321s, Critical Path: 0.21s
INFO: 1 process: 1 internal.
INFO: Build completed successfully, 1 total action
INFO: Running command line: bazel-bin/tools/repo/notify ./reviewers.yaml --dry_run
RepoNotifier NOTICE Slack message (#envoy-maintainer-oncall):
*Pending Deployments* (workflow runs that need manual approval from maintainer)
<https://github.com/envoyproxy/envoy-ci-staging/actions/workflows/request.yml?query=is%3Aaction_required>

[phlax]

thanks for this @nezdolik - small path issue then lets land it

[phlax]

lgtm, thanks @nezdolik

[botengyao]

waiting for #43521 and then can merge main.