Added a searchable input to the environments dropdown 1/5

What does this MR do?

As a part of improving the Monitoring dashboard environments dropdown the first step is to make it a searchable dropdown. This MR adds search input and filters the dropdown items based on the input.

Note: This is behind the feature flag searchable_environments_dropdown

This is the first MR for #34021 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team