Add shared ownership to tstring's VIEW type.

This change introduces `tstring::owner<T>`, a reference-counted wrapper for an object `T`. `tstring` can now be assigned a "shared view" using `assign_as_shared_view`, where the underlying data is owned by a `tstring::owner`. The `tstring` will increment the owner's reference count and decrement it when the `tstring` is deallocated or reassigned, ensuring the owner object remains alive as long as any `tstring` holds a shared view to its data. This is implemented by adding a `TStringOwnerCApi` pointer to the `TF_TString_View` struct and updating the relevant `TF_TString` functions to handle the reference counting.

PiperOrigin-RevId: 832101409

Author: LarryLansing

jax_tpu_embedding/sparsecore/lib/core/input_preprocessing.cc

@@ -418,7 +418,7 @@ void PopulateOutput(TableState& state, PreprocessSparseDenseMatmulOutput& out,
                     absl::Mutex& output_mutex) {
   state.stats_per_host.Flatten();
 
-  absl::MutexLock mutex_lock(output_mutex);
+  absl::MutexLock mutex(output_mutex);
   out.lhs_row_pointers[state.stacked_table_name] =
       std::move(state.csr_arrays_per_host.row_pointers);
   out.lhs_embedding_ids[state.stacked_table_name] =