Set right ownership for created files

[alegrey91]

Currently harpoon needs root privileges to run.
When we run harpoon, this creates several files with root ownership, making them unaccessible by the user who ran the command to create them.
We should add the ability to create these files with the ownership of the user that ran the command.

[alessio-perugini]

We can use the following syscalls to drop the sudo privilege:

• syscall.Setuid(1000)
• syscall.Setgid(1000)

Main problem:

• If we run harpoon as sudo, we are not able to see the uid of the user who invoked it. Calling the os.Geteuid() will return 0.

Possible solutions:

• After invoking the necessary code as sudo, we could drop the privilege to be nobody or set it to be uid:1000.
• Make configurable the uid/gid that we want to use when writing files. Through flags/envs.