feat: add phase RX gate

[math411]

• [ x ] I have added the tests to cover my changes.
• [ x ] I have updated the documentation accordingly.
• [ x ] I have read the CONTRIBUTING document.

Summary

Adds in the PhaseRx gate.

Details and comments

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2 out of 3 committers have signed the CLA.

:white_check_mark: math411
:white_check_mark: speller26
:x: Coull

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