Update Vitess Connector documentation

A few places in the Vitess documentation needs to be updated:

• `The key’s payload will always be null at the moment` is no longer the case, because now we've added support for extracting PK from Vitess event.
• Document the column naming convention, which is no column name should be named with a `@` prefix
• Document how to deal with the Vitess `@1` column bug error in the `Behavior when things go wrong`
• Document Vitses Connector static gRPC authentication configuration