AXIS2-6103 Add streaming JSON message formatters for large HTTP responses

New package org.apache.axis2.json.streaming with three classes:

- FlushingOutputStream: wraps transport OutputStream, flushes every
  64KB (configurable). Prevents reverse proxy 502 Bad Gateway on
  large responses by converting a single buffered body into a stream
  of HTTP/2 DATA frames or HTTP/1.1 chunks.

- JSONStreamingMessageFormatter: GSON variant. Drop-in replacement
  for org.apache.axis2.json.gson.JsonFormatter.

- MoshiStreamingMessageFormatter: Moshi variant. Drop-in replacement
  for org.apache.axis2.json.moshi.JsonFormatter.

Both formatters use try-with-resources for JsonWriter lifecycle
(addressed in Gemini code review). FlushingOutputStream uses long
for bytesSinceFlush counter to prevent int overflow on 2GB+ writes.

Enable globally in axis2.xml:
  <messageFormatter contentType="application/json"
      class="org.apache.axis2.json.streaming.MoshiStreamingMessageFormatter"/>

Optional per-service flush interval tuning in services.xml:
  <parameter name="streamingFlushIntervalBytes">131072</parameter>

No service code changes required. Tested on WildFly 32 locally and
behind a reverse proxy on stg-rapi02 (HTTP/2 ALPN). All existing
services (BigDataH2Service, FinancialBenchmarkService) produce
bit-identical JSON results.

Also includes:
- json-streaming-formatter.xml: full user guide
- toc.xml: cross-referenced in JSON (23.6), HTTP/2 (18.6), and
  MCP (23.4) sections
- springbootdemo-tomcat11 README: streaming formatter section