Add support for nested gRPC callouts.

[andytesti]

Panic with message "proxy-wasm-rust-sdk/src/dispatcher.rs:121:14:\nalready borrowed: BorrowMutError" is triggered when dispatch_grpc_call() is invoked from on_grpc_call_response(). The problem is originated by a self.grpc_callouts.borrow_mut() being invoked inside an if condition, that keeps the RefMut alive until the true branch ends.
This PR adopts the same approach than on_http_call_response(), by invoking the RefCell::borrow_mut() outside the if condition.

[PiotrSikora]

Thanks! This is a great find (and something that we've missed in #56, EDIT: because it predates gRPC support added in #100 and #101).

However, this fix is incomplete, since it only covers on_grpc_receive() callback, and only for gRPC calls and not for gRPC streams.

Could you also fix the case for nested gRPC streams in on_grpc_receive() and for nested gRPC calls and streams in on_grpc_close()?

[andytesti]

Thanks! This is a great find (and something that we've missed in #56, EDIT: because it predates gRPC support added in #100 and #101).

However, this fix is incomplete, since it only covers on_grpc_receive() callback, and only for gRPC calls and not for gRPC streams.

Could you also fix the case for nested gRPC streams in on_grpc_receive() and for nested gRPC calls and streams in on_grpc_close()?

Thanks for reviewing this, @PiotrSikora. I just added fixes for nested gRPC streams and on_grpc_close() cases.

[PiotrSikora]

LGTM, thanks!