Turn bgrpc.NewServer into a builder-pattern, with a config-based
initialization, multiple calls to Add to add new gRPC services, and a
final call to Build to produce the start() and stop() functions which
control server behavior. All calls are chainable to produce compact code
in each component's main() function.
This improves the process of creating a new gRPC server in three ways:
1) It avoids the need for generics/templating, which was slightly
verbose.
2) It allows the set of services to be registered on this server to be
known ahead of time.
3) It greatly streamlines adding multiple services to the same server,
which we use today in the VA and will be using soon in the SA and CA.
While we're here, add a new per-service config stanza to the
GRPCServerConfig, so that individual services on the same server can
have their own configuration. For now, only provide a "ClientNames" key,
which will be used in a follow-up PR.
Part of #6454
46c8d66c31