Introduce StaticSiteEditorConfigService

What does this MR do?

This is Iteration 2 of a three-part iterative approach to implementing the Static Site Editor config file.

This second iteration will introduce StaticSiteEditorConfigService (replacing CombinedConfig), with a stub placeholder implementation of FileConfig which returns a hardcoded hash of {static_site_generator: 'middleman'}.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides

Availability and Testing

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

Review App Static Site Editor Link

https://gitlab-review-add-sse-co-c7k3ne.gitlab-review.app/gitlab-instance-administrators-e1083909/gitlab-self-monitoring/-/sse/master%2FREADME.md

Related Issues/Epics

• #241164 (closed)
• &4267 (closed)