Add user setting for additional building block templates

[rwmcintosh]

For now you can download OSP supported bb templates from the bb library.

Add an additional custom library URL that can be configured.

The library should be hosted by GH similar to the OSP library
The loader should support authentication in case the custom library is not open source.

Existing source
https://raw.githubusercontent.com/Open-Systems-Pharmacology/OSPSuite.BuildingBlockTemplates/main/templates.json

[PavelBal]

Ideally, support multiple sources.