[36467] Add API users form and schema endpoints

[oliverguenther]

• Create form POST /api/v3/users/form
• Update form POST /api/v3/users/:id/form
• Users schema /api/v3/users/schema
• Extend custom fields to UserRepresenter

Attribute conversion fixes

firstName and lastName of user were not correctly converted to the frontend. That resulted in validationErrors properties containing the backend keys firstname and lastname. This has been fixed.

Representable nil patch
A patch is added for representable to fix trailblazer/representable#234, since Setter is not being applied when the input is nil.

https://community.openproject.com/work_packages/36467

[ulferts]

Apart from what I noted down in code, these are the things I noted when using the API

• The name property is not described in the schema
• firstName and lastName are described as non writable
• The password property is described although it does not exist on the user resource
• Attributes turn up in the form's payload although they are not writable (e.g. name):

[oliverguenther]

The password property is described although it does not exist on the user resource

The password property is a write-only property so it's not contained in the payload. It can still be set and written as per the schema

[oliverguenther]

@ulferts I believe addressed all remarks

[ulferts]

• The property email has the key mail in the schema

• In the API description, the :id part is missing from the routes:
  

• In the API description, the schema example seems to be malformed:

• The endpoint for user update is wrong (lacking the {id} part)

[ulferts]

👍

Thanks for fixing the user API description along the way.