CSI: replace structs->api with serialization extension

The CSI HTTP API has to transform the CSI volume to redact secrets,
remove the claims fields, and to consolidate the allocation stubs into
a single slice of alloc stubs. This was done manually in #8590 but
this is a large amount of code and has proven both very bug prone
(see #8659, #8666, #8699, #8735, and #12150) and requires updating
lots of code every time we add a field to volumes or plugins.

In #10202 we introduce encoding improvements for the `Node` struct
that allow a more minimal transformation. Apply this same approach to
serializing `structs.CSIVolume` to API responses.

Also, the original reasoning behind #8590 for plugins no longer holds
because the counts are now denormalized within the state store, so we
can simply remove this transformation entirely.