x/telemetry: allow to use custom config download logic

[hyangah]

The current telemetry upload logic uses go mod download golang.org/x/telemetry/config@latest
to download the current upload configuration.

When the go command is the uploader, the go command can avoid this extra subprocess
by directly accessing its go mod download implementation. To enable this, we need a hook to register the custom implementation. For example,

package telemetry

type Config struct {
...
   // UploadConfig, if set, is used to retrieve the upload config.
   // Otherwise, the program downloads the config using `go mod download`.
   UploadConfig(ctx context.Context, version string) (cfg *UploadConfig, resolvedVersion string, _ error)
}