Add design_input.vue component

What does this MR do?

Isolates the input element used in design uploads into its own component. All existing functionality it retained.

Note that the implementation changes slightly: previously, the uploaded files were being retrieved from the element itself $refs.fileUpload.files. This MR instead uses the event emitted from the @change event event.target.files. Tests have been updated to reflect this.

Why?

#39452 (closed) will introduce a dropzone, where we need to use input element in the exact same way. Abstracting into a single component ensures that these behave the same way and avoids duplication and ensures that all uploads in design management share consistent behaviour and rules. This MR prepares us for the main MR for #39452 (closed).

Note: next MR, where this component will be used for the second time => !26139 (diffs)

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