Support github license resolution

The CLI tool should support automatic resolving and augmentation of license information, when it is missing, for components in an SBOM.

For components with an external reference to a GitHub repo it should be possible, in a lot of cases, to automatically resolve license information using the API.

This is already supported in the .NET implementation (https://github.com/CycloneDX/cyclonedx-dotnet/pull/39). But there is an obvious need for this to be available across ecosystems (https://github.com/CycloneDX/cyclonedx-gomod/issues/2).

Supporting this in the CLI tool shouldn't be much effort. But the first step would be to refactor it into the .NET library so both the CLI tool and .NET implementation can leverage the same methods.