Define states parameter in environments api interface

What does this MR do and why?

This merge request builds on the work from !87023 (merged) as explained by @shinya.maeda in #362070 (closed), to ensure states parameter is defined in environments API interface which wasn't the case despite being documented!

It was working fine, regardless, because params was passed to the EnvironmentsFinder which shouldn't be the case mainly because API users shouldn't be allowed to pass arbitrary params, and also to keep consistent with other endpoints.

More test cases were also added to cover the changes introduced here.

How to set up and validate locally

1. Start GDK locally, if it isn't up already: gdk start.
2. Make sure one of the projects on your local installation has multiple environments configured.

for valid state:

3. In your terminal, send a request to the /environments endpoint (using your preferred method of API authentication) as follows:

echo -e | curl -v -H "Content-Type: application/json" -H "PRIVATE-TOKEN: YOUR_TOKEN" localhost:3000/api/v4/projects/:project_id/environments?states=available

4. Validate that the response has status 200 and returns all available environments.

for invalid state:

3. In your terminal, send a request to the /environments endpoint (using your preferred method of API authentication) as follows:

echo -e | curl -v -H "Content-Type: application/json" -H "PRIVATE-TOKEN: YOUR_TOKEN" localhost:3000/api/v4/projects/:project_id/environments?states=test"

4. Validate that the response has status 400 with the following error message: states does not have a valid value.

MR acceptance checklist

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