[FR] provide metadata describing supported STLs

The gradle plugin needs to know what STLs are supported and where the libraries that need to be packaged are located. This is currently hard coded, and for the most part is now static, but since we are planning on removing the system STL it might be useful to encode this info in the metadata.

Start by locating the Gradle plugin's hard-coded information about supported STLs and the library locations that must be packaged. Determine how metadata should represent this information, including the planned removal of the system STL; done means the plugin can use that metadata instead of relying on the current hard-coded values.

Written by the indexing model from the issue text.