Expose Pipeline information in GraphQL

In relation to https://gitlab.com/gitlab-org/gitlab-ce/issues/47393 we need to expose Pipeline information in GraphQL.

Pagination

We will need to paginate the pipelines for a merge request, paginate the stages & builds for a pipeline.

We should also use keyset pagination as described in: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18584.

This seems to be the go-to for GraphQL, using connections

Filtering

Allow filtering pipelines by their state.