Add incomplete integration plugins to metadata.json as skip

[karenzone]

Release notes

[rn:skip] to leave this PR out of release notes

Step 1: Move integration plugins

This PR moves logstash-integration-internal and logstash-integration-zeromq plugins to plugins-metadata.json and configure them as default-plugins: false and "skip-list": true to set us up to make the next set of changes to docs tooling.

Next steps

• ToDo: Change docs tooling to remove skip logic for integration plugins.

Related: https://github.com/elastic/docs-tools/issues/106