Bug: filter_spec is typed as mandatory, but per RFC 8641 it's optional.

[rodonile]

According to RFC 8641 filter-spec needs to be optional, not mandatory:

https://github.com/network-analytics/NetCalyx/blob/main/crates/netconf-proto/src/yang_push/filters.rs#L140-L141

At the moment we fail deserializing payloads with such filters without any xpaths or subtree filters specified:

      <selection-filter xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-push">
        <filter-id>ALARMSietf100</filter-id>
      </selection-filter>