feat: buildkite plugin renovation (#1874)

This PR adds initial support for buildkite plugin renovation.

It supports `plugin-name` or `my/plugin-name` plugins, and fully specified semver versions only (e.g. `v1.3.2`). Currently it will always propose an upgrade to the latest version available, e.g. if current version is v1.3.1 and both v1.3.2 and v2.0.0 exist then v2.0.0 will be proposed. Looks for any yml file in the `.buildkite/` directory.

Closes #1869