Implement SSE config file processing

What does this MR do?

This is Iteration 3 of a three-part iterative approach to implementing the Static Site Editor config file.

This third iteration will replace the stub placeholder FileConfig implementation with a full implementation similar to Gitlab::WebIde::Config and its associated Entry classes, e.g. ...:Entry::Global and ...::Entry::Terminal.

There will still only be a single string config entry supported, static_site_generator, with a single valid value of middleman.

Does this MR meet the acceptance criteria?

Tasks

• Config file supports a single entry for static_site_generator, with a single valid value of middleman.
• If an invalid value is given (anything other than 'middleman'), user is redirected to root project page with a descriptive error message displayed.
• If no config file exists, the default value of middleman will be used
• If an empty config file exists, the default value of middleman will be used

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

NOTE: The review app SSE page is returning a 422, even though it works in the local GDK. Need to debug why.

https://gitlab-review-add-sse-co-voziz0.gitlab-review.app/root/no-config-file/-/sse/master%2FREADME.md

Related Issues/Epics

• #241164 (closed)
• &4267 (closed)
• Follow-up issues:
  • Optimize and remove duplication for GRPC calls during StaticSiteEditor::ConfigService.execute: #251014 (closed)
  • Define strategy for error communication in the Static Site Editor: #213285 (closed)