Support reductions over a subset of axes

[sd109]

Adds an additional axis field to the request payload and converts the x-activestorage-count response header from a single i64 to a Vec<i64>.

Adapts each reduction operation to explicitly handle each of the three distinct axis options:

• reductions over all data (i.e. flattened to a 1D array) when request payload does not contain axis key
• reduction over a single axis when payload contains axis: <int>
• reduction over multiple axes when payload contains axis: <list-of-ints>

Corresponding changes to the compliance test suite: stackhpc/s3-active-storage-compliance-suite#23