Add toMap and toJson methods to BuildInfo

[sbly]

JSON is done via ujson-circe as this is what's already being used in the project itself

[lefou]

Mill uses upickle for JSON serialization. The circe version is probably only present for some contrib module.

[lihaoyi]

I don't think we should hardcode a ujson dependency in the BuildInfo modules. If anyone wants to convert it to JSON, calling upickle.default.write(BuildInfo.toMap) is trivial, and adding a ujson dependency bloats jars and leads to classpath conflicts

[lefou]

+1 for only adding the toMap method. But leaving out the toJson.

Edited: Removed the idea about pre-rendered JSON

[sbly]

@lihaoyi @lefou Thanks for the reviews! I removed the toJson method as recommended.

[lefou]

Thank you!