Migrate Nightly Benchmarks from captest to staging.
- Migrate GitHub Workflows
- Replace `zenith-benchmarker` with regular runners
- Remove `environment` parameter from Neon GitHub Actions, add
`postgres_version`
- The only job left on captest is `neon-captest-reuse`, which will be
moved to staging after its project migration.
Ref https://github.com/neondatabase/cloud/issues/2836
Add `neon-branch-create` / `neon-branch-delete` to allow using branches
in tests.
I have a couple of use cases in mind:
- For destructive tests with a big DB, we can create the DB once in
advance and then use branches without the need to recreate the DB itself
after tests change it.
- We can run tests in parallel (if there're compute-bound).
Also migrate API v2 for `neon-project-create` / `neon-project-delete`
* github/actions: add neon projects related actions
* workflows/benchmarking: create projects using API
* workflows/pg_clients: create projects using API
