Add ability to specify targets to profile

[kakkoyun]

This issue is about bringing back the functionality specified by --pod-label-selector and --systemd-units flags we had from the last major release.

      --pod-label-selector=STRING
                                  Label selector to control which Kubernetes
                                  Pods to select.
      --cgroups=CGROUPS,...       Cgroups to profile on this node.
      --systemd-units=SYSTEMD-UNITS,...
                                  [deprecated, use --cgroups instead] systemd
                                  units to profile on this node.

It's removed as part of #627, and the fully featured filtering will be added eventually.

Tasks

• As an initial implementation, let profiler to capture profiles but just do not send them to the server
• Specify pods to profile
• Specify systemd units to profile
• Specify any target to profile
• Reject the events in eBPF level if the PID is not as part of a desired target

Related #7

#887 covers almost all of the intended behavior, so we don't need half-measures for now. We need to update the documentation and provide samples of configuration to demonstrate the functionality is equivalent to the flags.

[maxbrunet]

I discussed with @kakkoyun, he will add back the flags before the next release, while I work on the long-term solution (#7) which may not be ready/stabilized early enough, I'll try to keep the flags as deprecated for backward compatibility

[kakkoyun]

Closing for the sake of #887