Schema/Config endpoints to modify configset with no core/collection

All schema/config configurations should work even in cases where a collection is not associated with them
This jira will involve
1. Refactoring existing handler/manager to work without SolrCore
2. Adding /api/cluster endpoints to support such modifications
Endpoints -

• /api/cluster/configset/{name}/schema
• /ap/cluster/configset/{name}/config