internal/gomote/protos: add a list swarming builders endpoint

This change adds a GRPC endpint to the gomote server which
lists all of the LUCI swarming builders for the project. The
builders names will be used to request the types of gomote
instance requested.

Updates golang/go#61773
For golang/go#61772

Change-Id: I2c6bfb3d251967b8008ba64ee28f2e1c8472df5b
Reviewed-on: https://go-review.googlesource.com/c/build/+/518796
Reviewed-by: Dmitri Shuralyov <[email protected]>
Run-TryBot: Carlos Amedee <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: cagedmantis