add socket handler for `view_submission` of the modal

[Len4i]

Description

Hi,
I'm using socket_handler middleware and it is really convenient to register actions this way. So i got to a point that i have several modal views with submit button and I need to react differently. For now there is no level 3 function to register reactions on submit requests, so need create some level 2 wrapper func that will handle all submit requests and dispatch them based on callback_id.

I've prepared some example: commit
I just recently started to dig into slack APIs and slack-bots in general, as well as I'm new to go, so feel free to raise concerns/point to cons in my proposal/fix it.
And thanks for slack-go :)

(Optional) Slack's documentation

in the bolt documentation also mentioned similar approach of differentiation submit request by callback_id

To listen to view_submission requests, you can use the built-in view() method. view() requires a callback_id of type str or re.Pattern.