[ENHANCEMENT] Introduce `blockstoredpairs`

Introduce `blockstoredpairs` to iterate over pairs of block locations and stored blocks, similar to [`SparseArraysBase.storedpairs`](https://github.com/ITensor/SparseArraysBase.jl/blob/b4f5a370d70d3bd1cfd872ea8187a6fe1e60ecf1/src/abstractsparsearrayinterface.jl#L76-L77).

_Originally posted by @lkdvos in https://github.com/ITensor/BlockSparseArrays.jl/pull/16#discussion_r1910401071_
            