provide Interceptor examples

[cruvie]

when I use try to use Interceptor, it takes me much time the expected, and AI cannot do it well, too.

there are many question issues about the usage of Interceptor, and I found there's no example in https://github.com/grpc/grpc-dart/tree/master/example

would you like provide some examples about Interceptor cover common usages?(like set jwt token and catch grpc errors)

and examples may append to the end of the doc https://grpc.io/docs/guides/interceptors/