expose Channel type and FUSE transport type via Thrift listMounts

Summary: FuseChannel already tracks its transport type (“devfuse” or “io_uring”) via getTransportName(), but this information was only used internally for telemetry logging. This diff adds two new optional fields to the MountInfo Thrift struct — fsChannelType (e.g., “fuse”, “nfs3”, “prjfs”) and fuseTransport (e.g., “devfuse”, “io_uring”) — and populates them in the listMounts() handler. This makes the transport type queryable by CLI tools and other Thrift clients without requiring log inspection.

Reviewed By: genevievehelsel

Differential Revision: D106560639

fbshipit-source-id: 02a9246fed054e1ab57cc0cc13651b0d92c49a73