feat: support project level custom fields

[Zaimwa9]

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Changes

Closes #6620

Support project scoped custom fields which could only be defined at organisation levels

• Added project as foreign key to MetadataField
• Replaced unique_together with conditional UniqueConstraints
• _validate_required_metadata accepts project param
• is_required_for fetched and included in responses directly (replacing 2 different requests made in the frontend => coming in next branch)
• Added pagination

New endpoint

• GET /projects/{id}/metadata/fields/ with
  • ?include_organisation to include inherited org-level fields (calculates the precedence for project in case of conflicts)
  • ?entity=feature|segment|environment to only return fields of the given identity (simplifies the creation of the identity)

⚠️
In case of conflicting name, project custom fields takes precedence and org ones are skipped

How did you test this code?

• Tests and following PR

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Project	Deployment	Actions	Updated (UTC)
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Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-e2e:pr-6736	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api-test:pr-6736	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api:pr-6736	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-6736	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-6736	Finished ✅	Results ✅

[gagantrivedi]

⚠️
In case of conflicting name, project custom fields takes precedence and org ones are skipped

Wouldn't it be less confusing to not allow conflicts?

[Zaimwa9]

@gagantrivedi

Wouldn't it be less confusing to not allow conflicts?

I do remember asking the question while grooming and we agreed on allowing re-creating the custom fields at project level.
Maybe @asaphko you can give us some product-wise insights here. To summarize, in project settings, we show as read-only the organisation field. You can create the same field for your project and this one will take over. A use-case I can see is to have a global custom field that you can require or not at project level.

What Gagan suggests is to simply forbid and return an error if the name already exists in the organisation (be it project/org level)

The base branch was changed.