feature: webhook for changes in the merkle tree #99
Description
/api/webhooks/add - a POST of { path:string, currentOxenRevision:string, purpose:string, contact:email } (no bearer token needed)
If the caller doesn't know the currentOxenRevision for the node in question they're told "no"
On Change: { path:string } is informed (no details of the change, just that some change has happened.
There would be obligations about performance expectation on the caller end of the web-socket. Oxen-server would keep some stats, perhaps. It might also have a policy as to where connections can come from. Oxen-Server would make no commitment to do the webhook onChange for every change - it may skip some notifications, it may send an onChange when nothing has changed. The notified node would perhaps call back after reply regular HTTP. It could also perhaps do nothing with the info.
The notified node may follow with a directory traversal (following a merkle tree to the subset of changes it is interested in. As for all webhooks, the list of to-be-notified may get dropped, and it is up to caller to re-establish the webhook if it thinks that's needed. Oxen-server will attempt to de-dupe the list of notify-ees but makes no guarantees.