[r] [Feature request] Implementas generics for SOMASparseNDArrayReader

[pablo-gar]

Is your feature request related to a problem? Please describe.
Low-level API SOMASparseNDArrayRead adds complexity to the general R user.

see discussion here #1274 (comment)

Describe the solution you'd like

The current SOMASparseNDArrayRead supports iterators for arrow Table and Matrix dgTMatrix, to get a full concatenated result users need to perform $read()$tables()$concat() and $read()$sparse_matrix()$concat().

The following convenient wrappers should be provided for a more R-like experience

as(x, "CsparseMatrix")
as(x, "TsparseMatrix")
as(x, "RsparseMatrix")
as_arrow_table(x)

where x is a SOMASparseNDArrayRead. This can allow for a functionality like this:

as_arrow_table(SOMASparseNDArray$read())

# equivalent to
SOMASparseNDArray$read()$tables()$concat()

cc @mojaveazure