Add JSON output format for remoting

[michaellwest]

Summary

Relates to #1374 — CliXml serialization via System.Management.Automation.Serializer is the main performance bottleneck in SPE remoting. This adds a -OutputFormat Json option as a faster alternative.

Problem

ConvertTo-CliXml uses reflection to walk the entire object graph with deep type information. For large result sets (hundreds of Sitecore items), this dominates response time. The existing -Raw switch bypasses serialization but returns unstructured .ToString() output.

Solution

New -OutputFormat parameter (CliXml | Json | Raw) on the remoting pipeline:

• Server (RemoteScriptCall.ashx.cs): Parses outputFormat query param, adds JSON branch using ConvertTo-Json -Depth 3 -Compress with {"output":[], "errors":[]} envelope, sets Content-Type: application/json
• Client (Invoke-RemoteScript.ps1): New -OutputFormat param on Invoke-RemoteScript, backwards-compat mapping from -Raw switch, JSON parse with fallback to CliXml for old servers
• Benchmark: Updated scenarios with 500 objects × 10 properties to stress-test serialization

Benchmark Results (500 objects × 10 properties)

Format	P50 (ms)	Avg (ms)
CliXml	596	827
Json	350	347
Raw	134	139

JSON is 2.4x faster than CliXml at P50 while preserving structured property data.

Backwards Compatibility

• -Raw switch continues to work unchanged
• Old client → new server: rawOutput=true maps to raw format
• New client → old server: Falls back to CliXml with a warning if JSON parse fails