Automatically allocate a new chunk group instead of throwing error due to buffer size limits

As in #5918, we might throw an error when writing into columnar storage due to several buffer size limits enforced by safestring library or by Postgres.

While the buffer size limit for `memcpy_s()` is 256MB, `memcpy()` doesn't enforce such a limitation; so #6419 attempts improving this situation by switching to use plain `memcpy()` instead `memcpy_s()`.
Note that even if we use `memcpy()`, memory allocators --such as `enlargeStringInfo()`-- still enforces a limit of 1GB when writing a chunk group into the disk.

Indeed, we could almost completely remove such memory limitations by deciding to allocate a new chunk group in `ColumnarWriteRow()` instead of throwing an error in the runtime (i.e., when trying to expand the latest chunk group based on GUC limits).