[plugin/grpc-transcode/pb_option/hooks] - How/where to set the concrete hook function[s] and what about the relevant use case?

[zhoujiexiong]

Currently, there are switches for the invocations of hook function, but

how/where to set the concrete hook function[s]?

There is no entry to do so at plugin config level, so

what about the necessity to enable_hooks/disable_hooks options or
what about the relevant use case?

see also - enable_hooks