feat(graphql-plugins): add ability for plugins to call back to core e…

[shirshanka]

…ngine

It is useful for plugins to be able to get common pieces from the core engine like existing loadableTypes etc, to extend the graphQL model easily.

Adding facilities for that.

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). If a new feature has been added a Usage Guide has been added for the same.
• For any breaking change/potential downtime/deprecation/big changes an entry has been made in Updating DataHub

[chriscollins3456]

looks good!

[shirshanka]

merging this because CI failures are unrelated.

[sachinsaju]

@shirshanka
The idea of GmsGraphQLPlugin is

An interface that allows the Core GMS GraphQL Engine to be extended without requiring
code changes in the GmsGraphQLEngine class if new entities, relationships or resolvers
need to be introduced. This is useful if you are maintaining a fork of DataHub and
don't want to deal with merge conflicts.

Now for adding this plugin to Datahub GMSGraphEngine, I have to add in the constructor of GmsGraphQLEngine

    this.graphQLPlugins = List.of(
            // Add new plugins here
    );

But this would in future cause merge conflicts right? Or is there any other way to add the plugin?