Create an StrEnum with the list of known entrypoint names

[chaen]

To avoid spelling mistakes

[aldbr]

• in the way we do extensions, we have e.g. policies, db extensions... based on entry points. These names are currently handled using Literals (constant strings). This should be converted into StrEnum.

[aldbr]

Need to go through the different subpackages (go through the entire codebase)

[ryuwd]

Not necessarily complete but the docs page I made in #701 might help

https://diracx--701.org.readthedocs.build/en/701/dev/reference/entrypoints/