Fix error from using `inputs` in some cases

What does this MR do and why?

A CI config linting error is being raised when using include:inputs with include:rules. It likely also occurs when matching include:inputs with other include subkeywords.

The fix is to include inputs in the allowed keys for the includes entry class.

Changelog: fixed

Issue: #436631 (closed)

Screenshots or screen recordings

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Before	After

How to set up and validate locally

1. Create a file named included.yml in a project's root directory. Give it this content:

   spec:
     inputs:
       stage:
   
   ---
   
   test:
     stage: $[[ inputs.stage ]]
     script: ls
   

2. In the CI editor, put this content:

   include:
     - local: included.yml
       inputs:
         stage: test
       rules:
         - if: $CI_COMMIT_BRANCH == "main"
   
   build:
     script: ls

3. See that the config has no errors