Introduce Internal API for searching environment names

What does this MR do?

This MR introduces an internal API for searching environment names from a given query.

End point	Acceptable params	Results
/*namespace_id/:project_id/environments/search	query (String, Required)	environment names (String) (e.g. ["review/patch-1", "review/patch-2"])

The returned values will be used as suggestions for users, like the following image.

We avoid using public API /api/v4/projects/:project_id/environments, because of performance reasons, for example,

1. It contains unnecessary attributes which could weigh on network bandwidth.
2. It uses pagination which could take way more entries than frontend requires.

Example

Query: http://localhost:8181/root/too-many-secret-variables/environments/search.json?query=review

Result: ["review/1","review/2","review/3","review/master_1","review/master_2"]

What are the relevant issue numbers?

Related https://gitlab.com/gitlab-org/gitlab-ee/issues/9439

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [-] Documentation created/updated via this MR
• [-] Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer