Support `GetDependencies` to consider "context" information

As [your own blog article](https://blog.deps.dev/zillions-of-sboms/index.html#downstream-consumers-control-the-composition) nicely explains, resolved dependencies depend on the context of the downstream consumer, i.e. the application. So when aiming to [create a "build-type-SBOM"](https://www.cisa.gov/sites/default/files/2023-04/sbom-types-document-508c.pdf), additional context has to be provided to the resolution mechanism, like the target framework fur NuGet applications, or the JDK version for Maven applications.

However, currently it seems the `GetDependencies` API cannot take any such context information, making me wonder what the default assumptions about the above-mentioned contexts are.

Ideally, the API would take `system`-dependent context parameters to allow the user to specify these explicitly.