Make TF_RESOURCE memory handling consistent with other types.

TF_Tensor's are backed by a contiguous memory region for all
but TF_RESOURCE tensors. The memory management of TF_RESOURCE
tensors required keeping a backing tensorflow::ResourceHandle*
object alive for the lifetime of the TF_Tensor object.

This change removes that discrepancy, making the memory backing
TF_RESOURCE tensors self-contained. This simplifies use of TF_RESOURCE
tensors in the C API (as users of the C API do not need to worry about
a tensorflow::ResourceHandle object and its lifetime). In doing so, this
moves a string memory copy from the TF_Tensor <-> Numpy conversion
to the C API from the Python session helper.

Unfortunately, I couldn't figure out how to add a simple unittest in
c_api_test.cc. The more comprehensive tensorflow/python/kernel_tests/session_ops_test.py
does cover the changed lines though.

Additionally, avoid an unnecessary copy when creating TF_STRING or TF_RESOURCE
tensors (as eigen alignment is not a requirement for them).

PiperOrigin-RevId: 163751880