API inputs endpoint should validate and inform whether inputs are available

[mabijkerk]

Problem
The current implementation of the inputs endpoint of the API extracts all the inputs that are available on ETSource. The issue is that users do not know whether they should use all these inputs:

• Some of those inputs however may be outdated and have long been removed from the frontend of ETModel. There is therefore no guarantee that these inputs still function properly. In some cases, specifying values for these inputs may cause unintended results, as in Remove aggregated inputs etsource#2839.
• Some of those inputs may only be relevant for certain specific modules, such as the external coupling module to the CTM, or specific inputs for the Holon project.

Solution
The goal of this enhancement is therefore twofold:

1. Allow ETEngine to validate whether inputs are available for a certain scenario
2. Present the user with information about this validation, and present information where relevant for other modules. An example may be to add an attribute that can attain the values available, unavailable and external module

@noracato I think there are multiple possible solutions for this issue. For example, we could also simply remove all inputs that are not marked as available, or we could remove all the inputs that are outdated from ETSource etc. I'm happy to brainstorm what solution would be best to implement.

Notifying @ChaelKruip

[github-actions]

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[mabijkerk]

See also quintel/etsource#2858