refactor: added a StrEnum for Entrypoints to avoid spelling mistakes

[Stellatsuu]

Closes: #622

Changes:

• added a strEnum EntryPointGroups in /diracx-core/.../extensions.py to avoid hardcoded strings and spelling mistakes

Questions:

• should /docs/reference/entrypoints.md match with https://diracx--701.org.readthedocs.build/en/701/dev/reference/entrypoints/#core-extension-registration? Should this new enum be documented anywhere?

Answer: #701 contains the doc and isn't merged yet

• does places where extension = diracx be replaced by EntryPointGroups.CORE? e.g:

# DiracX will always be there so force it to be the lowest priority
return sorted(extensions, key=lambda x: x == "diracx")
--> return sorted(extensions, key=lambda x: x == EntryPointGroups.CORE)

if installed_extension not in [None, "diracx"]: ...
--> if installed_extension not in [None, EntryPointGroups.CORE]: ...

cls.client_modules = ("diracx.client", f"{installed_extension}.client")
--> cls.client_modules = (f"{EntryPointGroups.CORE}.client", f"{installed_extension}.client")

Answer: we don't touch extensions