[Compute Separation] M3b: host.json delivery via gRPC

[brettsam]

In separated compute, the host cannot read host.json from disk — it lives in the worker's container. The worker sends it in StartStream.host_configuration_json. A custom IConfigurationSource blocks in ConfigureAppConfiguration until the worker provides it.

This works because the gRPC server starts at the WebHost level (via RpcInitializationService) before the ScriptHost is built.

Dependencies: M3 (#11645), PROTO (#11642)

Deliverables

File	Purpose
ExternalWorkers/HostJsonContentProvider.cs	WebHost singleton — SetContent() from gRPC, WaitForContent() for config pipeline, Reset() for restarts
ExternalWorkers/ExternalWorkerHostJsonConfigurationSource.cs	IConfigurationSource + ConfigurationProvider — blocks in Load()
ScriptHostBuilderExtensions.cs (modified)	Conditional swap of config source
Server/FunctionRpcService.cs (modified)	Intercept host_configuration_json from StartStream

Restart Handling

Scenario	Behavior
ScriptHost restarts, worker connected	Reset() keeps cache → next Load() immediate
Worker disconnects	Reset(clearCache: true) → next Load() blocks
Placeholder mode	Returns default config {"version":"2.0"} without blocking

Acceptance Criteria

• host.json from StartStream is parsed and applied as configuration
• All downstream option bindings (ScriptJobHostOptions, etc.) receive correct values
• Placeholder mode returns defaults without blocking
• ScriptHost restart uses cached content when worker is still connected
• Worker disconnect clears cache
• HostJsonFileConfigurationSource is unchanged
• Unit tests