batches: add GraphQL query for batch spec executions

Created by: courier-new

The backend work for #23032 (closed).

Adds new query to the GraphQL schema for batchSpecExecutions, which for now just takes basic pagination parameters. I'll be adding an additional set of tests for batch_spec_executions_connection.go, I'm just painfully slow at writing tests in Go right now (I think it's taken me longer to write the test than to implement the connection itself 😅), and I really wanted to get this up before the weekend.

Hoping to wrap up that test (in this PR) and the frontend list view (in a separate PR) on the plane or during our down time next week.