Rewrite v2 core-admin APIs using annotation framework

Solr's v2 APIs can be implemented in 1 of 2 possible ways: an apispec (i.e. JSON file) based approach that was originally conceived when the v2 APIs were created, and an approach that relies on annotated POJO objects which has come into favor more recently as it results in less duplication and inches our APIs ones step towards a more strongly-typed future.

The consensus has emerged across several JIRAs that the annotated-POJO approach is the one that should be used going forward, and that existing v2 APIs should be cut over as convenient.

It's worth noting that this will cause the introspection output to lose the "description" text for these APIs and their parameters, as there's no support for this yet for annotation-based v2 APIs. See SOLR-15117 for more details.

This ticket aims to tackle this conversion for the v2 equivalents of the v1 "Core Admin" APIs