Project-level integration: Implement project-level setting selector

Description

As part of the iteration 1 #213857 (closed), we need to adjust the behavior of the project-level integration page.

We have to implement project-level frontend selector and backend behavior:

• Use instance level settings, set inherit_from_id = instance.id
• Use custom settings, set inherit_from_id = nil

In any case, we have to pre-populate the values with the instance-level values.

➡ Prototype: low-fidelity overview
➡ Prototype: high-fidelity detailed flow

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