[FEA] golang API for index serialization back to CPU

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All indexes like cagra, ivf_flat, ivf_pg, ... have similar issue that is without serialization functions to CPU in golang APIs.

e.g. golang ivf_flat API only hav 3 functions CreateIndex, BuildIndex and SearchIndex. There is
no function to serialize the index back to CPU just like C api does.

C APIs

/**
 * @brief De-allocate IVF-Flat index
 *
 * @param[in] index cuvsIvfFlatIndex_t to de-allocate
 */
cuvsError_t cuvsIvfFlatIndexDestroy(cuvsIvfFlatIndex_t index);

/** Get the number of clusters/inverted lists */
cuvsError_t cuvsIvfFlatIndexGetNLists(cuvsIvfFlatIndex_t index, int64_t* n_lists);

/** Get the dimensionality of the data */
cuvsError_t cuvsIvfFlatIndexGetDim(cuvsIvfFlatIndex_t index, int64_t* dim);

/**
 * @brief Get the cluster centers corresponding to the lists [n_lists, dim]
 *
 * @param[in] index cuvsIvfFlatIndex_t Built Ivf-Flat Index
 * @param[out] centers Preallocated array on host or device memory to store output, [n_lists, dim]
 * @return cuvsError_t
 */
cuvsError_t cuvsIvfFlatIndexGetCenters(cuvsIvfFlatIndex_t index, DLManagedTensor* centers);

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Complete the golang API to match with C, Rust and Python for all indexes.

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Additional context
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Start by comparing the Go IVF-Flat entry points CreateIndex, BuildIndex, and SearchIndex with the C API functions and the existing Rust and Python APIs. Done means the Go API provides serialization-back-to-CPU and matching index accessors for all indexes, including the C API's destroy, list-count, dimension, and centers operations.