SOLR-15187: v2-POJO based SolrRequest classes

[gerlowskija]

Description

The POJOs used for serialization/deserialization by some V2 APIs have a lot of duplication with the existing SolrRequest objects for those APIs.

Solution

Since they share the same fields, SolrRequest objects can be implemented using their v2-POJO counterparts.

Tests

None, yet.

Checklist

Please review the following and check all that apply:

• [x] I have reviewed the guidelines for How to Contribute and my code conforms to the standards described there to the best of my ability.
• [x] I have created a Jira issue and added the issue ID to my pull request title.
• [x] I have given Solr maintainers access to contribute to my PR branch. (optional but recommended)
• [x] I have developed this patch against the master branch.
• [ ] I have run ./gradlew check.
• [ ] I have added tests for my changes.
• [x] I have added documentation for the Ref Guide as appropriate (for Solr changes only).