feat: create custom fields for projects

[Zaimwa9]

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Changes

Closes #4733

Implements the frontend to create and manage project-level custom fields (taking precedence over organisation fields)

• Following nesting model_fiels, replaced the 2 requests + merge approach with a single call to projects/{id}/metadata/fields/
• Removed mergeMetadataFields 😌
• added getProjectMetadataFieldList with pagination
• added project to create project-scoped custom fields
• ProjectSettings custom fields page now shows organisation custom fields (read only) and a new project-related custom fields table
• project_custom_fields flag to hide the custom fields table for now

How did you test this code?

https://www.loom.com/share/5747526237e64f95b45683f251fc04ad

[talissoncosta]

LGTM! Thanks for handling that @Zaimwa9

The base branch was changed.

[Zaimwa9]

@talissoncosta heads-up here that I will remove the feature flag because:

• It lost its interest following refactoring the requests (it's not gating the requests)
• it's mostly an enterprise feature, and it would add friction to FoF users to have to activate it themselves

[talissoncosta]

could you please fix the conflicts @Zaimwa9 ?

[talissoncosta]

Hey @Zaimwa9! Great work on it.

I just noticed some type inconsistencies with projectId and organisationId across the metadata components. I created a follow-up PR to clean them up: #6817