Tokens expire after 1 hour - use Tokensource instead.

https://github.com/GoogleCloudPlatform/golang-samples/blob/42a8597d5beb089c313eb618ac1363af5e404be0/run/grpc-ping/connection.go#L30

The current example manually adds a token to each client request. For some longer running services, this becomes an issue since the client is usually instantiated when launching the service, requests made after 1 hour of the initial client instantiation will fail since the token will have expired by then.

Rather use the TokenSource methodology where the underlying token is validated and only refreshed when needed.

Here is a revised `NewConn` method illustrating a potential solution to the above:
```go
type grpcTokenSource struct {
oauth.TokenSource
}

// NewConn creates a new gRPC connection.
// host should be of the form domain:port, e.g., example.com:443
func NewConn(ctx context.Context, host string, insecure bool) (*grpc.ClientConn, error) {
var opts []grpc.DialOption
if host != "" {
opts = append(opts, grpc.WithAuthority(host))
}

if insecure {
opts = append(opts, grpc.WithInsecure())
} else {
systemRoots, err := x509.SystemCertPool()
if err != nil {
return nil, err
}
cred := credentials.NewTLS(&tls.Config{
RootCAs: systemRoots,
})
opts = append(opts, grpc.WithTransportCredentials(cred))
}

// use a tokenSource to automatically inject tokens with each underlying client request
audience := "https://" + strings.Split(host, ":")[0]
tokenSource, err := idtoken.NewTokenSource(ctx, audience, option.WithAudiences(audience))
if err != nil {
return nil, status.Errorf(
codes.Unauthenticated,
"NewTokenSource: %s", err,
)
}
opts = append(opts, grpc.WithPerRPCCredentials(grpcTokenSource{
TokenSource: oauth.TokenSource{
tokenSource,
},
}))

return grpc.Dial(host, opts...)
}
```