feat: add support for specialization of models through subtypes with …

[swaroopjagadish]

…looker integration

This PR

• Adds a new aspect called SubTypes to allow for specializing of the base models (like Datasets).
• Allows this aspect to be added to the Dataset Entity without needing to be added to the DatasetSnapshot model
• Adds support for Looker views and explore ingestion to emit this new aspect, while being compatible with old servers that don't have this aspect yet
• Adds support in the UI to use the subtype information to specialize the rendering

Checklist

• The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
• Links to related issues (if applicable)
• Tests for the changes have been added/updated (if applicable)
• Docs related to the changes have been added/updated (if applicable)

[shirshanka]

LGTM! This looks pretty neat.