Enable developers to customize the way to handle unmatched requests

[seratch]

Currently, there is no way to customize the behavior of App when an incoming request does not match with any listeners in the app. To improve this, we can extract these lines of code as a default behavior and enable developers to override the function in some way.

As we already have methods like oauth*Error, I'm thinking to name the new method as unhandledRequestError or something like that I went with unmatchedRequestHandler in AppConfig

Unlike other Bolt frameworks (JS / Python), Bolt for Java relays runtime exceptions to the external layer like Bolt adapters and its runtime. If a developer would like to handle the unmatched patterns in a single error handler, the recommended way is to throw an exception in the unhandledRequestError (the name is work in progress) method. I went with unmatchedRequestHandler in AppConfig

See also:

• Enable developers to customize the built-in receivers more bolt-js#860
• Enable developers to customize the way to handle unmatched requests  bolt-python#273

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

• bolt (Bolt for Java)
• bolt-{sub modules} (Bolt for Java - optional modules)
• slack-api-client (Slack API Clients)
• slack-api-model (Slack API Data Models)
• slack-api-*-kotlin-extension (Kotlin Extensions for Slack API Clients)
• slack-app-backend (The primitive layer of Bolt for Java)

[seratch]

In this improvement, we can add something similar to slackapi/bolt-python#323 too.