Extract some functionality to make writing custom App/Receiver easier

[seratch]

Description

To enable developers to implement their own Receiver like https://github.com/seratch/bolt-aws-lambda-proof-of-concept , we need to extract some functionalities in ExpressReceiver and expose them to 3rd parties.

Those works should be based on @barlock 's PRs for v2.

• Convert say, ack, and respond to return promises #369
• Compose middleware #375
• [v2] Convert receivers to use App#processEvent #380
• [v2] Add documentation for async middleware and App#processEvent #381

What type of issue is this? (place an x in one of the [ ])

• bug
• enhancement (feature request)
• question
• documentation related
• testing related
• discussion

Requirements (place an x in each of the [ ])

• I've read and understood the Contributing guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've searched for any related issues and avoided creating a duplicate issue.

[barlock]

I think it's time to make bolt a monorepo too?

I'd be interested in helping maintain a KoaReceiver after all those go in.

I've also been developing one for testing with jest.

[seratch]

@barlock
At least in the short term, we're not planning to have multiple modules in this project. My intention here is to enable any 3rd parties to build Bolt extensions more easily.

Let me share what I want to extract from App and ExpressReceiver in detail. Some of the code are already outdated though.

From App, I would like to extract these.
https://github.com/seratch/bolt-aws-lambda-proof-of-concept/tree/master/src/added/app
By doing this, the onIncomingEvent part could be much simpler. Developers don't need to repeat the same. https://github.com/seratch/bolt-aws-lambda-proof-of-concept/blob/master/src/added/TwoPhaseApp.ts

From ExpressReceiver, I want to extract the following after refactoring them to be independent from Express.

• https://github.com/seratch/bolt-aws-lambda-proof-of-concept/blob/master/src/added/requset-body-parser.ts
• https://github.com/seratch/bolt-aws-lambda-proof-of-concept/blob/master/src/added/signature-verifier.ts

The extraction allows me to easily implement my own Receiver (TwoPhaseReceiver is a sub-interface of Receiver) like this:
https://github.com/seratch/bolt-aws-lambda-proof-of-concept/blob/master/src/added/AwsLambdaReceiver.ts

@barlock
Are you interested in implementing some of these ideas? If so, I'm happy to work together! But don't feel pressured at all. I was going to start working on this. I just wanted to know if you were also planning to do the similar.

In the case I'll work on this, my pull request on top of your commits will be coming early next week. I'd appreciate it if I could have your reviews on it. 🙏

[barlock]

It's all yours! I appreciate the offer, and I appreciate more you basing it off my commits. 🤞 Here's hoping it get's reviewed soon.

Maybe we can finally close #81 ! What a long 3 year wait it's been 😛

[seratch]

I should have verified our v2 implementation and checked if we can close this issue but I haven't done yet. When I do those in the near future, I will update you here.

[seratch]

While working on #670, probably we can resolve this issue.

[seratch]

I know there are still rooms for improvement but, with the latest version, writing a custom receiver is much easier now. Let me close this issue I raised here.