Add pre and post invocation hooks

[ejizba]

Fixes #522. Importantly, this PR does not address all possible hook-related features, but it should allow for them in the future (hopefully near future). See that issue thread for more details on that discussion.

Sample usage:

import { registerHook } from '@azure/functions-core';

registerHook('preInvocation', (context: PreInvocationContext) => {
    // can completely change the inputs passed to a user's function
    context.inputs = ['hello'];
    // can write to `context.hookData` and read the value in a postInvocationHook
    context.hookData.value = 1;
    // can write to the _invocation_ context passed to a user's function
    context.invocationContext.dataForInvoc = 2;
});

const disposable = registerHook('postInvocation', (context: PostInvocationContext) => {
    // can read data from `context.hookData` set in the preInvocationHook
    if (context.hookData.value === 1) {
    }
    // can modify the error thrown by the function
    if (context.error) {
        context.error = new Error(`Add a prefix to error text: ${context.error.message}`);
    }
    // can completely change the result of a function
    context.result = 'world';
});

disposable.dispose(); // Can stop the hook from executing in the future

See workerTypes/index.d.ts for more details/docs. See here for why I picked the name I did and how I see this built-in module working in the new programming model. Ultimately, though, we could easily rename this module and do something completely different in the future.

[hossam-nasr]

For the interest of type safety, I would support adding separate types for pre and post invocation hook callbacks, and reflecting that here.

[ejizba]

I added the separate types, but I ran into too many build/lint errors if I tried to reference them here. If you can come up with a simple/clean way to do that let me know, otherwise I don't think this is worth the effort

[hossam-nasr]

Yeah I gave it a try but I couldn't get them to work either, without having to use different functions to retrieve/execute each hook. I guess it's another argument for having separate functions 😋

[ejizba]

Thanks Hossam! Most of your comments are topics I've debated myself, so I'm glad to be ironing them out with someone else now 🙂

[ejizba]

Met with the app insights team today. Overall looks good to them, but we need to add the function callback to the hooks api somehow. Filed #553 to address separately