[mlir][sparse] Rename add{Pointer,Index} to append{Pointer,Index}

This clarifies that these methods only work in append mode, not for general insertions.  This is a prospective change towards https://github.com/llvm/llvm-project/issues/51652 which also performs random-access insertions, so we want to avoid confusion.

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D120929