Make it easier to match FlightSQL messages

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
When implementing FlightSQL the do_get and get_flight_info gRPC methods are called for several of the message types. Thus those endpoints need to decode the incoming message and then do the appropriate action

However, the current way the messages are decoded from prost::Any means the only way I have figured out how to check dynamically for each message type is an if chain like this

        } else if let Some(decoded_cmd) = Any::unpack::<CommandGetCatalogs>(&msg)? {
          // act
        } else if let Some(decoded_cmd) = Any::unpack::<CommandGetDbSchemas>(&msg)? {
          // act
        } else if let Some(decoded_cmd) = Any::unpack::<ActionCreatePreparedStatementRequest>(&msg)?
        {
          // ...
        }

It would be great to have an enum so a rust match statement could be used

Describe alternatives you've considered
@stuartcarnie proposed the following in https://github.com/influxdata/influxdb_iox/pull/7213/files#r1137963627:

I created a gist that extends the sql crate in arrow-flight, that demonstrates an approach allowing us to remove all the if Some(..) to a single match:

match Commands::unpack(&msg)? {
    Commands::CommandStatementQuery(cmd) => Ok(Self::CommandStatementQuery(cmd)),
    Commands::CommandPreparedStatementQuery(CommandPreparedStatementQuery { prepared_statement_handle }) => Ok(Self::CommandPreparedStatementQuery(preparedstatementhandle::try_decode(prepared_statement_handle)?)),
    // etc
}

I extended the prost_message_ext macro to create a Commands enum with all the messages passed to the macro. The Commands macro has a static function, unpack, that is capable of unpacking any of the registered messages.

Additional context

[alamb]

cc @avantgardnerio

[avantgardnerio]

This would be awesome to solve, thanks @stuartcarnie !

[stuartcarnie]

@avantgardnerio and @alamb figured I'd clean up the Gist and open the PR:

• feat: Add Commands enum to decode prost messages to strong type #3887

[tustvold]

label_issue.py automatically added labels {'arrow'} from #3887