Add support for custom routes to HTTPReceiver & SocketModeReceiver

[stevengill]

Description

Similar to how ExpressReceiver allows defining custom routes, it would be nice if HTTPReceiver and SocketModeReceiver did the same.

This has been requested a few times before. See #834

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

• enhancement (feature request)

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.

[smoneta]

Im also having issues (lack of knowledge more likely) add ing custom routes in socket mode. I added the code and I'm working on migration the expressReceiver routes I have in order to use them under the SocketModeReceiver. The sample code in the bolt page is great but would be nice to include a sample endpoint too.

Thanks.