NuGet local repositories should support API Search() by targetFramework

Work on NEXUS-6158 has been focused on solving the immediate issue of broken builds caused by lack of support for the targetFramework attribute. This issue represents the remainder of the work no longer encompassed by that issue, including:

1. Capture of targetFramework attributes for hosted repos in a way that they can be easily used in queries from the Search() API endpoint. This should likely entail consideration of how to represent compatible targetFramework versions per the NuGet specification as part of the data model.

2. Implementation of the Search() api method's targetFramework query parameter in a way consistent with the NuGet specification, including the aforementioned issues of determining compatible targetFrameworks.

3. As we're not capturing this information yet in searchable form, we may need to implement a means of migrating data from the dependencies attribute (now containing the targetFramework data in the format expected by NuGet) to the new targetFrameworks attribute we'll need to use for searching. If we don't do something, users that start utilizing NuGet hosted following NEXUS-6158 will end up with gaps in returned results (or they'll have to redeploy their artifacts to pick up these changes, which may or may not appear unseemly).