feat: automatically set GOMEMLIMIT

[dunglas]

Setting the Go runtime memory limit can improve significantly the performance of Caddy (and FrankenPHP and Mercure, according to our tests), especially on low-resources machines and containers.

This patch uses github.com/KimMachineGun/automemlimit to automatically find a good limit using the currently configured groups. This is especially useful when setting limits to containers using Docker, Kubernetes, LXC, and the like.

This PR is similar to #6047, which does the same things go the max number of processors.

Thanks to @flovntp for the idea and for finding this library!

[mholt]

Thanks Kevin! Do you think we could add a comment to this code (and maybe it could be shared with the gomaxprocs code above it) since I don't think it's obvious why it's useful? Then we will merge!

[dunglas]

@mholt, done.

I also added logs for GOMAXPROCS to improve the overall experience. Commands now start with something like this:

2025/01/26 21:11:59.838 INFO    maxprocs: Leaving GOMAXPROCS=10: CPU quota undefined
2025/01/26 21:11:59.838 INFO    GOMEMLIMIT is updated   {"package": "github.com/KimMachineGun/automemlimit/memlimit", "GOMEMLIMIT": 15461882265, "previous": 9223372036854775807}