all: refactor .pb.go generation (grpc#3451)

Replace various //go:generate lines and regenerate.sh scripts with a
single, top-level regenerate.sh that regenerates all .pb.go files.

Placing generation in a single script ensures that all files are
generated with similar parameters. The new regenerate.sh uses the
protoc-gen-go version defined in test/tools/go.mod and automatically
handles new .proto files as they are added.

Do some minor refactoring on .proto files: Every file now has a
go_package option (which will be required by a future version of the
code generator), and file imports are all relative to the repository
root.

Author: neild

balancer/grpclb/grpclb.go

@@ -16,8 +16,6 @@
  *
  */
 
-//go:generate ./regenerate.sh
-
 // Package grpclb defines a grpclb balancer.
 //
 // To install grpclb balancer, import this package as:

balancer/grpclb/regenerate.sh

@@ -16,8 +16,6 @@
  *
  */
 
-//go:generate protoc -I grpc_testing --go_out=plugins=grpc:grpc_testing grpc_testing/control.proto grpc_testing/messages.proto grpc_testing/payloads.proto grpc_testing/services.proto grpc_testing/stats.proto
-
 /*
 Package benchmark implements the building blocks to setup end-to-end gRPC benchmarks.
 */