Add benchmark for lock file size

[Hofer-Julian]

Big lock files can become quite a problem, as discussed in prefix-dev/pixi#1509.
So we want to make sure we want to keep track of it.

The following benchmarks would be interesting:

• lock file size
• compressed lock file size
• git repo size with multiple commits containing lock file changes

[tdejager]

Good points! For the last point it might be easier to set that using pixi, maybe automated with a python script.