Standardized way to make intra-cluster (REST) queries

### Problem
While we don't have nice Java API binding, as, for example, gRPC provides, intra-cluster queries are done in various ad-hoc ways, for example, using manual String formatting:

https://github.com/apache/incubator-druid/blob/984958122b962ab019a590f4fd01792de81b75df/server/src/main/java/org/apache/druid/client/DirectDruidClient.java#L150-L151

Or, constructing URLs in static methods:

https://github.com/apache/incubator-druid/blob/984958122b962ab019a590f4fd01792de81b75df/server/src/main/java/org/apache/druid/server/lookup/cache/LookupCoordinatorManager.java#L717-L735

This creates difficulty finding places in the code where a certain Druid API is called. Ideally, the experience should not be significantly worse than IDE's "Find usages" action.

### Proposed solution

Every REST method (`@GET`/`@POST`-annotated) which is called within the cluster has an accompanying static method for forming an URL to call the method from a remote node. This accompanying static method is placed in a centralized place, such as `IntraClusterCalls` class. Javadocs for the REST method and the accompanying static method link to each other for easy navigation. All intra-cluster calls should be made using these static methods for creating URLs.

Start by reading the intra-cluster URL construction in DirectDruidClient.java and LookupCoordinatorManager.java, then compare those callers with the REST methods they invoke. Review the proposed IntraClusterCalls class and the linked REST endpoints. Done means intra-cluster calls use centralized URL-building methods and the REST methods and helpers link to each other in Javadocs.

Written by the indexing model from the issue text.