[MM-54232] Implement automatic client registration

[streamer45]

Summary

To make the service play nicely with K8S (or with any other form of load balancing of multiple instances) we added some very hacky logic in mattermost/mattermost-plugin-calls#479 to essentially re-attempt actions due to failed authentication. Not only was this pretty horrible code to implement but also it didn't work particularly well with round-robin load balancers which would distribute each request to a different backend, causing the client to just give up after reaching the maximum failed attempts.

To alleviate these issues and simplify the flow we are adding some functionality to automatically register a client (if missing) upon authentication attempt. This has the huge benefit of removing the need to make separate HTTP requests which could all hit different endpoints. Instead, both registration and authentication are now coupled with whatever action the client is making (e.g. starting a job). This means that a call with valid credentials is guaranteed to succeed, no matter which endpoint it hits.

Related PR

mattermost/mattermost-plugin-calls#501

Ticket Link

https://mattermost.atlassian.net/browse/MM-54232

[cpoile]

This kind of makes me wonder if there's any point to registering/authentication if self registration is allowed. :)

[streamer45]

This kind of makes me wonder if there's any point to registering/authentication if self registration is allowed. :)

I think there still is some value in a multi-tenant scenario since you'd like to know which client is authenticating but for self hosted not that much.