Add Strategy Component for Feature Flags

What does this MR do?

As part of the redesign of feature flags to allow a single strategy to apply to multiple environments (#35555 (closed)), a new component that acts as a form for a strategy is introduced.

Strategy - Handles setting the strategy type, as well as any necessary parameters to go along with that strategy.

New Environments Dropdown - A new dropdown that searches and autocompletes on any environment scopes for feature flags, very similar to the currently existing Environments Dropdown.

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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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