[Feature] Add Python Package Builder Support for FastAPI and APIRouter EntryPoints

[deeleeramone]

This PR adds capabilities to PackageBuilder and the Python static assets:

• support for EntryPoints that are instances of fastapi.FastAPI and fastapi.APIRouter.
• improved standardization in ReferenceGenerator.

Additionally, it removes logic in openbb-build that attempted to rely on downloading. The module is now part of openbb-core, so this is unnecessary.

The new scenarios have been added to the testing suite.

You can test this by creating a router extension, both EntryPoints can be in the same file.

[tool.poetry.plugins."openbb_core_extension"]
empty = "openbb_empty_router.empty_router:diff_router"
also_empty = "openbb_empty_router.empty_router:router"

On openbb-build, this should result in:

Extensions to add: also_empty@0.0.1, empty@0.0.1