Add build.act.json for storing dependencies

[plajjan]

This is as a POC and getting-going kind of thing. Should weave into #1841 etc later.

Anyhow, we need to store dependency information somewhere and I think it would be good if it can easily be read AND written programmatically, thus a simple JSON file!

To begin with, this is not used by build.act, it will be used by the vanilla project build. The first step, to support zig package dependencies, we just need to write the information into build.zig and build.zig.zon

We need to know the URL / hash to fetch. I think it would be good with a name (we can generate one for the zig dependency code but a human understandable is nicer). We also need the names of the artifacts to link with / install headers from. That should be a list of strings per dependency. For example mbedtls exposed three separate libraries.