Log drains need ability to customize logs sent to endpoints

[evelant]

Currently log drains have entirely fixed output. That means we cannot transform logs in any way which makes it difficult to work with the logs in other services. Specifically in our case we drain logs to Sentry but have no way to associate those logs with user sessions in our app because we cannot set a trace id on the logs. The best we can currently do is add a header with a unique session id since that shows up in log metadata, but that's less than ideal since it requires manual querying on the sentry end in order to correlate anything.

I think log drains would be a lot more valuable if we could transform the logs before they're sent out and/or customize them in some way. This would provide the flexibility needed to work with formats external tools expect. Log drains are less useful when they make the assumption that everything else will consume logs in exactly the format Supabase sends, since almost none will, but that's the current state of things.