Add GHZ states as benchmarks

[nathanshammah]

I propose to add GHZ states as benchmarks. They are easy to encode and are a maximally entangled state.

They are present also in mitiq benchmarks, https://github.com/unitaryfund/mitiq/blob/main/mitiq/benchmarks/ghz_circuits.py and https://mitiq.readthedocs.io/en/stable/apidoc.html#module-mitiq.benchmarks.ghz_circuits

[nathanshammah]

https://www.linkedin.com/posts/jay-gambetta-a274753a_the-preparation-of-ghz-states-is-a-common-activity-7302459921470550016-kjGR

[nonhermitian]

Note that the above uses a particular method to get those results. The GHZ state here https://github.com/unitaryfund/mitiq/blob/main/mitiq/benchmarks/ghz_circuits.py is also a less than ideal way to generate the state for everyone except perhaps IonQ, who can only run gates in serial.