feat(attribute-completeness): improve attribute-completeness for single attribute

[Gigaszi]

Description

Corresponding issue

Closes #

805

New or changed dependencies

-

Checklist

• [x] I have updated my branch to main (e.g. through git rebase main)
• [x] My code follows the style guide and was checked with pre-commit before committing
• [x] I have commented my code
• [x] I have added sufficient unit and integration tests
• [x] I have updated the CHANGELOG.md

TODO

• [x] Let's put attribute metadata information also into /metadata as with the other metadata.
• [x] Fix figure (needle and figure heading)
• [x] Make attribute request parameter optional for other indicators. This request parameter should be required for the Attribute Completeness
• [x] Take a look at swagger → Swagger should not have Attribute Completeness indicator as example. Example should not container a attribute key.
• [x] Add Attribute Enum and use it pydantic request models to generate a clean OpenAPI spec
• [x] Add validation of topic-attribute combination to attribute-completeness

The UI-related tasks were moved to an issue in the Ohsome Dashboard project:

https://github.com/GIScience/ohsome-dashboard/issues/42

Some forward-looking backend-related tasks were moved to a new issue:

819