[smart_holder] Add a new return value policy `return_as_bytes` (#3838)

* Add return_as_bytes policy

* Fix format

* Fix test failures

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* Fix std::variant

* Resolve comments

* Note this policy experimental

* Add tests for `return_as_bytes` with `def_property`.

* Change comment for the new return_as_bytes enum to note that the policy is not available on master.

* Applying pr3838_sh.patch (exactly as used Google-internally since 2022-03-31).

* Add `case return_as_bytes` to `switch`es in detail/type_caster_base.h and eigen.h

Based on systematic review under https://github.com/pybind/pybind11/pull/3838#issuecomment-1094390333



* Add missing break (clang-tidy).

* More clang-tidy fixes (this time around clang-tidy was run interactively to pre-empt repeat trips through the CI).

* Underscore prefix: _return_as_bytes

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Ralf W. Grosse-Kunstleve <rwgk@google.com>