Add "list" command

[jgaskins]

Reported by @bendangelo here

TODO: Find and link to docs for this command — I've never heard of it before 😄

[bendangelo]

According to these docs. The sever should respond to "list".

https://github.com/fullstorydev/grpcurl

# Server supports reflection
grpcurl localhost:8787 list

[jgaskins]

That's not gRPC documentation. That's a third-party tool.

Looks like this is related to the concept of "reflection" in gRPC and, according to the gRPC docs, first-party libraries don't enable reflection automatically. I'm down with adding support, but that's an important distinction to make, especially for gRPC APIs exposed outside a VPC.