feat: CI/CD on Frappe Cloud

We should have a full CI/CD offering on Frappe Cloud for developers. A basic version of CI/CD on Frappe cloud can look like this:
- Devs will pay for some compute on a cheap upstream provider like Hetzner or bare-metal.
- Devs can configure github repositories on FC and integrate with FC using GitHub action (deploy)
- Whenever a PR is raised it will create a new image with those changes and copy of a test site will be spawned in that bench.
- Users can use it for manually testing

Few technical changes required to make this work IMO:
- Builds need to happen on same server. We can't build this in centralized fashion because PRs have very frequent changes.
- Asset building will require something like https://github.com/frappe/press/issues/4213 ( @Aradhya-Tripathi )
- Memory management needs to be done in some sane manner by implementing sleepy containers that get spawned on first request?
- Even the server itself should be created on demand and then archived to save costs.
- Fast DB cloning + migrate?
- ...

Do get in touch with me or @nikkothari22 for brainstorming since he has a lot of ideas on this and has worked in a setup that requires such CI/CD experience.

The issue names no files, tests, or entry points. Start by reviewing the proposed GitHub Actions integration and the related press#4213 asset-building work; define a scoped implementation and success criteria for builds, test-site creation, resource lifecycle, and database cloning before coding.

Written by the indexing model from the issue text.