Allow to select agent in environment setting page

What does this MR do and why?

As part of allowing to select an agent in the Environment page, this MR adds an agent selector to the environments settings page. The change is behind the environment_settings_to_graphql feature flag. Currently, we don't have an agent returned to the environment query, therefor the previous selection won't be visible until the next MR.

Screenshots or screen recordings

How to set up and validate locally

1. Enable the environment_settings_to_graphql feature flag
2. Visit Project -> Operate -> Environments (if the new navigation is enabled) or Project -> Deployments -> Environments (if the new navigation is not enabled)
3. Create/Edit an environment using the UI
4. Verify the agent selector is present
5. Select preserve log in the Chrome network tab to see the GraphQl mutation after the page update
6. Verify the clusterAgentId field is sent to the environmentCreate and environmentUpdated mutations.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

• UX review
• frontend developer review
• frontend maintainer review

Related to #396407 (closed)