Add serialization support to more gates

[NoureldinYosri]

This PR adds serializatoin support to

• IdentityGate => fixes #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 #6418)

supersedes #6458

cc: @dstrain115 @maffoo

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

:exclamation: 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

```diff @@ 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.

[NoureldinYosri]

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