Add `channel.ad_break.begin` for EventSub

[Aluerie]

Description

Hey, I'm Irene Adler❤️ from discord, coming from a small discussion in #help - just threw together some implementation for channel.ad_break.begin subscription type for Event Sub.

The link to twitch dev docs: https://dev.twitch.tv/docs/eventsub/eventsub-subscription-types/#channelad_breakbegin

Checklist

• [X] If code changes were made then they have been tested. Little proof:
  • [X] I have updated the documentation to reflect the changes.
  • [ ] I have updated the changelog with a quick recap of my changes.
• [ ] This PR fixes an issue.
• [X] 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, ...)
• [X] I have read and agree to the Developer Certificate of Origin for this contribution