Add a TypeSpec agreement bot

My understanding is that we want a message like:

- Swagger is deprecated, and your service will have to migrate to TypeSpec before MonthName 202X
- We encourage you to start your conversion process asap following the guidance of aka.ms/IwantTypeSpec to avoid any delay in future releases
- We will NOT grant exception to release if you're still using Swagger after the deadline. Your service will be tagged out of compliance for SDK release.
- In order to merge the current Swagger PR, you need to explicitly write as a comment "@typespec-req agree" to confirm you've read this text, and are aware of the requirements of moving to TypeSpec.
- You will have to explicitly agree every 3 months until your service is in TypeSpec

From a technical standpoint:

- If PR author already is "agree list" or PR is TypeSpec -> return
- Write a comment with the vetted text
- React on @typespec-req agree to add author to agree list