feat: add callbacks support

[lesyk-lesyk]

This PR utilizes the code from #1102 written by @Jonathan-Bailey-Bose and #757 written by @Makashov to add callback support to ReDoc.

• Updated layout for Callbacks middle section:

  • Callbacks by default will use the operation summary (or description, if summary does not exist) when rendered by ReDoc instead of callback name
  • Added method budge (POST / PUT / etc.) for callback summary
  • Added Deprecated label for deprecated callbacks (with strikeout callback summary)
    

• Created new component for visualizing Callback details

  • Show description in Callback details
  • Move Endpoint component from the right panel to Callback details (added compact property for Endpoint component for displaying smaller HTTP method verb)
    

• By default, the callbacks do not use top-level Servers
  

• But servers can be defined

  • on the path level
    
  • or operation level
    

• Added one more dropdown for switching callback request examples (with truncating long names with an ellipsis). Created a "GenericChildrenSwitcher" component that tracks the selected item itself.
  

• Added new endpoint /store/subscribe and few callbacks in openapi.yaml demo file with different configs to illustrate all possible configurations for callbacks.

• Also, in the scope of this PR was updated styles for the label in the dropdown for consistency and added gray colors to the theme.

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All together:

[RomanHotsiy]

LGTM in general.

I left a few minor comments.

Also,

• could you make color of this shevron not so dark?
  
• rename Callback request samples to Callback payload samples

[RomanHotsiy]

🎉

Thanks @Jonathan-Bailey-Bose and @Makashov for initial implementation!

Kudos to @okachynskyy for finishing it up 👏 🔥!

I will publish a new release at the end of the week.

[RomanHotsiy]

Released in 2.0.0-rc.27