gitaly_client: Use `ListAllLFSPointers` to retrieve all LFS pointers

What does this MR do?

The function BlobService#get_all_lfs_pointers() is still using ListLFSPointers to list all LFS pointers which exist in the repository. This function is highly inefficient given that it has to perform a walk of the object graph to find all LFS pointers, whereas we can just iterate over all objects directly without doing a graph walk.

Convert the function to use ListAllLFSPointers instead to improve performance.

Part of #337505 (closed).

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Conformity

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Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)