Add path for extra RO-Crate profiles

[douglowe]

This PR adds functionality to provide extra profiles for the rocrate-validator to use. This is via the extra_profiles_path, as this enables users to provide either a single new profile, or to overwrite all the profiles provided by the rocrate-validator.

When the user provides profiles already present, the validator will run, but just give a warning along the lines of:

[2026-01-19 14:41:21,317] WARNING in models: Profile with identifier workflow-ro-crate-1.0 at                                                                                                
 /Users/user/work/manchester/eScience/HDR-FedA/Cratey-Validator/venv/lib/python3.11/site-packages/rocrate_validator/profiles/workflow-ro-crate is being overridden by the profile loaded from                   
 ../../RO-Crate-Tooling/rocrate-validator/rocrate_validator/profiles/workflow-ro-crate.                                                                                                     

Still to-do:

• docker compose update
• extend tests to cover situations where we add extra profiles?

Side effects of this PR:

• I've cleaned up the config class, and set it so that we use this class for initialising the celery instances.
• I also (thank you chatgpt) added a more explicit check for existence of environment variables.
• The requirement for the celery environment variables is relaxed, matching the previous setup. However this is now explicit, so it looks a bit odd that we don't mandate these important variables. If I make these required some tests fail though, and I've not explored yet why that is.

[douglowe]

I have been stymied by inconsistent behaviour from the --extra-profiles-path flag. See this issue raised with CRS4: crs4/rocrate-validator#143

[douglowe]

I've added an integration test, which will fail until the validator is updated.