Store HarmonyRelease URL properties as string instead of URL

[kellnerd]

I am considering to abandon using the URL type for properties of HarmonyRelease in favor of plain strings, which are easier to handle in a number of scenarios:

• There can be serialization/deserialization issues with dynamic Fresh components: An URL property may silently turn into string properties when they are rendered on the client-side
• It becomes easier to have a JSON API later when everything is JSON already (which URL is not)
• No more issues when using structuredClone during merging
• Smaller test snapshots without redundant URL serializations

The main disadvantes which I see are:

• Less type safety: A string is not guaranteed to be a valid URL
• There are cases where we have to construct an URL from the string again to use e.g. the domain
• It is a breaking change which requires changes to all providers and test snapshots

This should affect external links, API links and image URLs only.

Please let me know if you see any other potential downsides or have alternative ideas.