Investigate V2RequestSupport's role in SolrJ's V2 API support

The V2RequestSupport interface was the original plan for building support for the v2 API into SolrJ's existing request objects. It relies on CollectionApiMapping.Meta to convert v1 request objects into instances of the generic V2Request class.

This worked well as the V2 APIs were originally conceived, but has developed a few issues over time.

1. V2RequestSupport's reliance on the Meta mapping class means that it only works with jsonspec-based v2 APIs and not the annotated-POJOs that the community intends to use going forward for defining V2 APIs.
2. It requires users to create request objects that rely on v1 APIs and concepts. e.g. there is no "Collection Admin" path in v2 APIs.
3. Usage of the interface never took off. It's only currently used by CollectionAdminRequest. So it's somewhat abandoned already.

We should deprecate V2RequestSupport in 8.x and plan to remove it in 9.0. In parallel (and in different JIRAs), we can work on adding v2 support to SolrJ one SolrRequest class at a time. (Classes that expose v2-compatible abstractions can be switched over to v2 implementations under-the-hood. Classes that don't (e.g. CollectionAdminRequest) can be deprecated and replaced by a more v2-friendly SolrRequest implementation.)