Missing support for *receiving* RPC marshaled objects with optional interfaces

[AArnott]

Outbound RPC marshalable objects have a chance to declare their optional interfaces by way of this interface:

vs-servicehub/src/servicebroker-npm/src/jsonRpc/MarshalableObject.ts

Lines 57 to 60 in 2d5a870

	export interface RpcMarshalable {
	readonly _jsonRpcMarshalableLifetime: MarshaledObjectLifetime
	readonly _jsonRpcOptionalInterfaces?: number[]
	}

But inbound RPC marshaled objects only generally satisfy this interface:

vs-servicehub/src/servicebroker-npm/src/jsonRpc/MarshalableObject.ts

Lines 120 to 133 in 2d5a870

	export interface MarshaledObjectProxy extends IDisposable {
	_jsonrpcMarshaledHandle: number
	}
	export module MarshaledObjectProxy {
	export function is(value: any): value is MarshaledObjectProxy {
	const valueCandidate = value as MarshaledObjectProxy | undefined
	return typeof valueCandidate?._jsonrpcMarshaledHandle === 'number'
	}
	}
	interface MarshaledObjectProxyTarget extends MarshaledObjectProxy {
	messageConnection: MessageConnection
	}

Note the lack of the _jsonRpcOptionalInterfaces property. This property becomes important for the receiver when the original object may support specific interfaces.

We should expose this property on the proxy target.