Multiple project homes

[hoijui]

Problem

As of now, we allow to set a single value for repo (URL) ,
and a single value for release (repo-relative path or URL) .

This means, that if a project has multiple homes -
for example the 3D models on github and the manufacturing instructions on a wiki -
there is no way to specify that with OKH,
apart from explicitly linking to specific files (hosted on these platforms).

This leads to intents like:
https://github.com/PubInv/OSMS-OKHs/blob/8ea8502b18d28a6a8d941b53a7e3b4f23923f51c/Air-Movement-and-Climate-Control/3DP-Protective-Cases-for-Aranet4-CO2-Monitor.okh.toml#L14

-> very reasonable and understandable, even good: more data and more thorough and accurate data about the project.
yet we evaluate such a manifest as invalid.

Solution Proposal

Similar to what we did for images,
we would convert the repo field into a complex one,
and at the same time we would also allow multiple values.

That allows us to have multiple values and yet define the primary/main one.
It would also open the door for possible solutions to #160,
once we reach a conclusion of how to handle that.