Add Buildkite pipeline checks

Add plugin to run Buildkite checks. For example:

1. Verify that all strings that don't need a double quote don't have one
2. Verify that all build steps in a certain pipeline have a certain element defined (e.g. `github_commit_status`).

Context: https://github.com/woocommerce/woocommerce-ios/pull/13872#issuecomment-2344280592

Start by examining the repository's existing Danger plugins and the Buildkite pipeline configuration described in the issue. Define the scope around checking unnecessary double quotes and required build-step elements such as github_commit_status; done means both checks can report violations for a pipeline.

Written by the indexing model from the issue text.