cmd/gomote: get list of buildlet types from server
cmd/gomote: get list of buildlet types from server

So users don't need to update their binaries all the time.

Fixes golang/go#30929

Change-Id: I53a6a092578ec0bcee3e47a5dcbbb5c0b9ee9113
Reviewed-on: https://go-review.googlesource.com/c/build/+/169678
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/build,golang/build,golang/build,golang/build