NestBot calendar events support

**Is your feature request related to a problem? Please describe.**

As per @bkimminich's suggestion on today's Project Leaders Town Hall we should consider implementing a feature that would allow users to configure upcoming meetings/events notifications for a specific Slack channel using Google calendar as a source.

**Describe the solution you'd like**

The feature is quite substantial and likely would have been part of the GSoC 2025 scope if we had come up with the idea earlier. While it's a good candidate for [OWASP Nest sponsorship program](https://github.com/OWASP/Nest/discussions/909) we can start working on baby steps toward implementing it already today (brainstorm, discuss, design -- you know -- our regular workflow).

**Are you going to work on implementing this?**

- [ ] Yes
- [x] No

**Additional context**

I'd start from assessing the feasibility of this request and move forward based on the research results.