callbackUrl - allow server to reject/restrict callbacks it is willing to make

[brong]

6.2 Web hook

The JMAP server MUST follow any redirects.

Maybe I'm a bit paranoid, but I suggest: The JMAP server
implementation MUST be capable of following at least one redirect, but
MAY choose not to do so for configured policy reasons.

6.2.1. setPushCallback

I think this needs another error code:

policyDenied: The server is not willing to connect to the specified
URL for policy reasons.