Introduce configuration value for image upload path

What does this MR do?

Add a new value to the Static Site Editor configuration file called image_upload_path.

Possible values: (string), default source/images

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required) (docs for initial config file introduction are being deferred to a separate issue)
• Code review guidelines
• Merge request performance guidelines

Availability and Testing

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

Related Issues and MRs

• Depends on (and rebased on, until merged) !43384 (merged)
• Related #241165 (closed)