feat: Username for process_exec events

[anfedotoff]

Username is useful when tetragon works on host. On different hosts the same username can have different UIDs. Approach based on resolving username with pure Go os/user LGTM. But there is some restrictions:

• /etc/passwd is available
• tetragon agent and process for which UID is resolved are in the same mount/user namespace

TODO:

• [x] Check if tetragon agent and process for which UID is resolved are in the same mount/user namespace
• [x] Username resolving by a flag (maybe?) // I think flag is not needed.
• [x] Test // Add username check in TestEventExecve
• [x] Docs // Field description exists in reference. Maybe we need to find some place in docs to describe restrictions of username resolving

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[anfedotoff]

@jrfastab, please, have a look. I think the PR is ready.

[jrfastab]

hi :wave: sorry for the delay I'm going to look at it in more detail tomorrow quick scan looks good to me.

[anfedotoff]

@tixxdz , please, have a look. I think PR is ready:).

[tixxdz]

The CI failures seem unrelated