Add option to spawn processes as siblings

[jprendes]

This PR adds the option to spawn processes as siblings of the calling processes instead of spawning them as children.
See #3011 for more context.

[utam0k]

Is there a good way to test this behavior?

[jprendes]

I think we can create an integration type of test, and check the ppid of the init process. I'll give that a go later today.

[jprendes]

@utam0k, I´ve added two integration tests, one to check that it can run as a child, and one to check that it can run as a sibling.

@YJDoc2, I don't think this is a breaking change, as it´s adding a new method, not removing or changing an existing one.

[YJDoc2]

@YJDoc2, I don't think this is a breaking change, as it´s adding a new method, not removing or changing an existing one.

My apologies. I saw that the builder was a pub struct, and we were updating the fields, so it was breaking. But we actually don't expose the fields, only the builder methods, so this should not be a breaking change. Have removed the label.

[YJDoc2]

looks good to me 👍
Maybe wait for utam0k 's reply, but approving from my side.

[utam0k]

Thanks!