feat: add plugin support for downstream pipelines

[jortega0]

This PR adds the ability to provide the plugins keyword to individual downstream buildkite jobs. This is needed so that we can use other plugins for downstream jobs that monorepo-diff-buildkite-plugin is using.

    plugins:
      - monebag/monorepo-diff#v2.5.8:
          ...
          watch:
            - path:
                - "foo/**"
              config:
                ...
                plugins:
                  - seek-oss/aws-sm#v2.3.1:
                      env:
                        AUTH_SECRET:
                          secret-id: "secret/id"
                          json-key: ".key"
            - path:
                - "bar/**"
              config:
                ...
                plugins:
                  - seek-oss/aws-sm#v2.3.1:
                      env:
                        AUTH_SECRET:
                          secret-id: "secret/id2"
                          json-key: ".key2"

See #142 for more information on the problem.

• TODO: Add unit tests
• TODO: Update Readme examples

[mbarrien]

This PR is also necessary to get https://github.com/buildkite/agent-stack-k8s working in combination with monorepo plugin. Please review soon!

[anherrera]

Any update on getting this shipped? My org could benefit from this! Thank you :)