Load OSADL Matrix at Runtime

[jens-erdmann]

As discussed in person (with @sschuberth) already, it would be a helpful improvement to enable the loading of the OSADL matrix at runtime.
The current implementation downloads the latest version of the matrix from upstream and integrates it at build time, as I understood the implementation.
There could be a use case for loading an older version of the matrix or a modified one at runtime. This could also pave the way for alternative providers of license compatibility matrices, which is what we intend to use it for first.

[sschuberth]

For the record, this could be useful in combination with tools like https://github.com/hesa/elmat.