[Canvas] Migrate expression runner from bfetch to an HTTP route

[lukasolson]

Summary

As part of getting ready to remove the bfetch plugin (see #186139), we need to move consumers of the bfetch services off of it.

Currently, Canvas utilizes bfetch for running an expression:

kibana/x-pack/plugins/canvas/server/routes/functions/functions.ts

Lines 54 to 67 in aa3b89f

	bfetch.addBatchProcessingRoute(API_ROUTE_FUNCTIONS, (request) => {
	return {
	onBatchItem: async (fnCall: FunctionCall) => {
	const handlers = {
	environment: 'server',
	};
	const result = await runFunction(handlers, fnCall);
	if (typeof result === 'undefined') {
	throw new Error(`Function ${fnCall.functionName} did not return anything.`);
	}
	return result;
	},
	};
	});

This needs to be moved into a regular HTTP route before we can remove the bfetch plugin.

[elasticmachine]

Pinging @elastic/kibana-presentation (Team:Presentation)

[elasticmachine]

Pinging @elastic/kibana-data-discovery (Team:DataDiscovery)