Bash completion for RPC names

It would be a really nice enhancement to the usability of the tool if you could auto-complete the names of RPCs. I often find it difficult to type out the `..` fully from memory and end up having to use several `list` commands to jog my memory before my actual intended command. I'm imagining this would look like:

```
$ grpcurl -plaintext localhost:50051
# when the user presses tab, shows a list of service names
myapp.LogsService
myapp.UsersService

$ grpcurl -plaintext localhost:50051 myapp.LogsService
# when the user presses tab, shows a list of RPC names
myapp.LogsService.ListLogs
myapp.LogsService.DownloadLogs

$ grpcurl -plaintext locaclhost:50051 myapp.LogsService.List
# when the user presses tab, auto-completes to myapp.LogsService.ListLogs
```

I did a quick search and haven't seen this proposed before but apologies if I missed any duplicates.

The main challenge as far as I can see is populating the available entries for completion -- the scenario I've described above would require server reflection, and even then it _could_ be slow if you make a reflection request on every tab so there might be some tricks needed.

Is this something that would be appreciated as an addition to the tool? I'm happy to attempt making a PR (with my sharply limited Go expertise) if so.