Add serialization support to more gates

[NoureldinYosri]

This PR adds serializatoin support to

• IdentityGate => fixes Cannot serialize identity gate on Quantum Engine #4833
• HPowGate => allows sending circuits with cirq.H to backend
• changes the handling of SingleQubitCliffordGate from automatic conversion to PhasedXZGate to have its own proto (originally proposed in Support serialization of SingleQubitCliffordGate #6418)

supersedes #6458

cc: @dstrain115 @maffoo

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.75%. Comparing base (4513892) to head (5b93695).

❗ Current head 5b93695 differs from pull request most recent head 010012f. Consider uploading reports for the commit 010012f to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6479      +/-   ##
==========================================
- Coverage   97.76%   97.75%   -0.02%     
==========================================
  Files        1105     1105              
  Lines       94963    94959       -4     
==========================================
- Hits        92837    92823      -14     
- Misses       2126     2136      +10     

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[verult]

You might already thinking about this as a next step, but reminder to add to the device gateset as well. Happy to talk through the required changes if needed.

[dstrain115]

Same comment as before w.r.t. numpy serialization. In this case, I think it might be okay, but we should at least document it.

[NoureldinYosri]

I changed the serialization to be independent of numpy

[NoureldinYosri]

@verult thanks for pointing that out. that will be a seprate PR if we need it.