Add support for launch_activity interaction response

[Joosemi02]

Summary

Activities can now be launched in response to interactions using the LAUNCH_ACTIVITY (type 12) interaction callback type for APPLICATION_COMMAND, MESSAGE_COMPONENT, and MODAL_SUBMIT interaction types.
https://discord.com/developers/docs/change-log#recent-api-updates

• Added the response type launch_activity (12)
  https://discord.com/developers/docs/interactions/receiving-and-responding#interaction-response-object-interaction-callback-type

• Created launch_activity method for InteractionResponse

Checklist

• If code changes were made then they have been tested.
  • [/] I have updated the documentation to reflect the changes. (I added docstrings)
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

[Joosemi02]

While I know that Activities are out of scope, I think this allows for compatibility with many bots that just want to have a command to launch an Activity and that way they can do it directly from the library.

I tested it and it works just fine launching the activity.

I found a thread about support for Entry Point Command (⁠bikeshedding⁠ > Entry Point Command) but it doesn't necessarily need to be added for this to work

[Rapptz]

This is fine, it's just missing the .. versionadded:: 2.6 string in the method docstring.