[SPIKE] New manifest to help build predictable URLs

[alexluckett]

Context

When a user uploads a file, we poll the backend to check the file upload status at /{prefix}/upload-status/{uploadId}. However, prefix is not known to the client-side JS and may even be missing if a developer doesn't register forms-engine-plugin with a prefix.

Problem

[ This is no longer actually a problem as I fixed the immediate issue in main, however my fix is brittle ]

Currently, our file-upload JS breaks if the plugin doesn't have a prefix. So we need better logic to determine the "base path" of the plugin.

Solution

Introduce a new manifest plugin that sits on a fixed URL, giving client-side JS a predictable path to build URLs by using the config it returns.

buildUploadStatusUrl shows why this is useful.

Pros:

• Allows us to build predictable URLs
• Means URLs aren't brittle, e.g. if the URL structure of file upload pages changed

Cons:

• Server registration options for engine are passed through as plugin registration options. Slightly unusual, although not major.
• Collissions if our plugin is registered multiple times. Moderately important, although I don't think anyone does this currently.

[sonarqubecloud]

Quality Gate failed

Failed conditions
0.0% Coverage on New Code (required ≥ 90%)

See analysis details on SonarQube Cloud