Add data transfer graphql queries under feature flag

What does this MR do and why?

Adds Active Record queries to Graphql resolvers for GroupDataTransfer and ProjectDataTransfer endpoints.
Adds feature flag data_transfer_monitoring_mock_data which allows user to ignore queries and use mock data.

Allows testing of query performance.

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/387026.

Screenshots or screen recordings

How to set up and validate locally

1. Use this snippet to generate table data for the project_data_transfers table.
2. Ensure the feature flag data_transfer_monitoring_mock_data is switched off.
3. Use GraphiQL to run queries locally on your GDK. Queries are the same as those described in the description of !110211 (merged)

MR acceptance checklist

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