AXIS2-6104 Move shared RPC classes to library-agnostic package

Moved Axis2JsonErrorResponse, JsonRpcFaultException, PaginatedResponse,
and PaginationRequest from org.apache.axis2.json.gson.rpc to
org.apache.axis2.json.rpc — a new shared package that neither gson
nor moshi imports depend on.

Before: moshi.rpc imported gson.rpc classes, creating a misleading
cross-dependency between two implementations that should be independent.

After:
  org.apache.axis2.json.rpc/       ← shared contract (no library imports)
  org.apache.axis2.json.gson.rpc/  ← Gson-specific receivers + JsonUtils
  org.apache.axis2.json.moshi.rpc/ ← Moshi-specific receivers + JsonUtils

Both gson and moshi receivers import from the shared rpc package.
The shared classes contain zero Gson or Moshi imports — they are
plain POJOs serializable by any JSON library.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>