- [X] Have you signed the [CLA](http://www.ubuntu.com/legal/contributors/)? ----- This PR implements the spec ISD160 - 12-Factor FastAPI Support" for rockcraft. For that, a new init profile called "fastapi-framework" and a new fastapi extension, called "fastapi-framework" are needed. These additions are similar to the flask and django init profiles and extensions, with several differences. Some of the main differences are: FastAPI and starlette supported. The ASGI entrypoint location can be in serveral places (and it is ASGI instead of WSGI). No dependency on gunicorn. FastAPI works with uvicorn without a configuration file. No statsd. Ubuntu 24.04 by default (supported bases 24.04 and bare). All current extensions (go, django, flask and now fastapi), have similarities (at the end they extend/change the rockcraft.yaml file), but also many subtle different details, like how they service command is overridden, extra tools or frameworks, what can be overridden and how, extra env variables (only Go).... As so, I have decided to not try to use a common base class for all of them (allowing them to easily evolve independently without a coupled inheritance structure). This is a tradeoff, as code is similar between frameworks. This is open for discussion.