Switch Perl library from JSON::XS to JSON::MaybeXS

The Perl library currently depends directly on JSON::XS, which forces a decision of the JSON backend on the user. Perl currently has a number of compatible backends, including JSON::PP, which is shipped with Perl itself.

The JSON::MaybeXS library serves as a compatibility layer to allow users to select the JSON backend that matches their stack, rather than forcing them to install a specific one, while still benefiting from the performance boost of XS libraries if they are available.

Note that AVRO-1514 already exists aiming to clean up some dependencies, including replacing JSON::XS with plain JSON, a different compatibility layer.

While the other changes in AVRO-1514 I think are desirable, that ticket seems to have stalled, and JSON::MaybeXS seems like a better alternative since it also supports Cpanel::JSON::XS as an additional possible backend.