Cast bytearray to string (#3707)

* Add bytearray to string cast, testcase and rename load_bytes to load_raw

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* New bytearray test case and convert failure to pybind11_fail

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* Fix merge comments

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* Assert early if AsString fails

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Porras Huang <pohuang@jackx-vm-1.nvidia.com>