Expose deployment frequency data through GraphQL endpoint

Proposal

Expose the deployment frequency data added in !48265 (merged) through GraphQL. Currently this data is only available through the REST endpoint.

Why?

The CI/CD Analytics page sources all of its data from GraphQL, except for deployment frequency data. This makes the frontend less consistent and will make future development on DORA 4 metrics slower and more challenging.

Technical details

This will not be as straightforward as some REST ⭢ GraphQL migrations, since much of the endpoint's logic is built directly into the API file, which GraphQL can't use.

We'll need to extract this logic into a shared location that can be used by both REST and GraphQL code.

Once this data has been exposed through GraphQL, we should update the deployment frequency graphs (implemented in #275991 (closed)) to use this new data.