Add native support for warmup trigger

[castrodd]

This PR is a fix for issues #174 and #176.

Currently, users wanting to set up a warmup trigger need to use generics. For example, they could write:

const { app, output, trigger } = require('@azure/functions');

app.generic('warmup', {
    trigger: trigger.generic({
        type: "warmupTrigger",
        direction: "in",
        name: "warmupContext"
    })
    handler: async (warmupContext, context) => {
        context.log('Function App instance is warm.');
    }
});

This works, but with the changes in this PR, users can now call the warmup trigger directly on app:

import { app, InvocationContext } from "@azure/functions";

export async function warmupFunction(warmupContext: object, context: InvocationContext): Promise<void> {
    context.log('Function App instance is warm.');
}

app.warmup('warmup', {
    handler: warmupFunction,
});

[charlesgardyn]

Could you tell me what happens when you try to deploy that using the Consumption plan? It'd be nice if there were a nice error message

[ejizba]

nit: Should be just WarmupContext without Options