feat: fastapi-framework init profile and extension

[javierdelapuente]

This PR implements the spec ISD160 - 12-Factor FastAPI Support" for charmcraft.

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.

For the fastapi profile, the base 24.04 is used.

[mr-cal]

Assuming the extension works, this looks good to me.

@lengau, do you typically write spread tests for extensions in charmcraft?

[lengau]

Good catch @mr-cal!

@javierdelapuente could you modify tests/spread/commands/init-flask-framework to test this?

As a rough step-by-step:

1. Rename the directory to init-extensions
2. Create flask and fastapi variants, setting the PROFILE environment variable to the relevant profile
3. Change the charmcraft init command to use --profile ${PROFILE}

[javierdelapuente]

Good catch @mr-cal!

@javierdelapuente could you modify tests/spread/commands/init-flask-framework to test this?

As a rough step-by-step:

1. Rename the directory to init-extensions
2. Create flask and fastapi variants, setting the PROFILE environment variable to the relevant profile
3. Change the charmcraft init command to use --profile ${PROFILE}

@lengau!, I have done it that way for all the missing frameworks (also go).

[lengau]

Looks great, thank you!