Add Snowplow Micro to the GDK

What does this Merge Request do and why?

Related to #1235 (closed)

This adds Snowplow Micro to be managed by GDK as a Procfile service, based on the snowplow-micro-configuration (which is currently used to run Snowplow in development) and how other GDK services were implemented.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.