Let caller set BRICKS_UPSTREAM for user agent (#196)

Example when called from vscode (and everything is hooked up):

```
> * User-Agent: bricks/0.0.21-devel databricks-sdk-go/0.2.0 go/1.19.4 os/darwin upstream/databricks-vscode
```

cmd/root/root.go

@@ -23,8 +23,11 @@ var RootCmd = &cobra.Command{
 	SilenceUsage: true,
 
 	PersistentPreRun: func(cmd *cobra.Command, args []string) {
+		ctx := cmd.Context()
+
 		// Configure our user agent with the command that's about to be executed.
-		ctx := withCommandInUserAgent(cmd.Context(), cmd)
+		ctx = withCommandInUserAgent(ctx, cmd)
+		ctx = withUpstreamInUserAgent(ctx)
 		cmd.SetContext(ctx)
 
 		if Verbose {

cmd/root/user_agent_upstream.go

@@ -0,0 +1,33 @@
+package root
+
+import (
+	"context"
+	"os"
+
+	"github.com/databricks/databricks-sdk-go/useragent"
+)
+
+// Environment variables that caller can set to convey what is upstream to bricks.
+const upstreamEnvVar = "BRICKS_UPSTREAM"
+const upstreamVersionEnvVar = "BRICKS_UPSTREAM_VERSION"
+
+// Keys in the user agent.
+const upstreamKey = "upstream"
+const upstreamVersionKey = "upstream-version"
+
+func withUpstreamInUserAgent(ctx context.Context) context.Context {
+	value := os.Getenv(upstreamEnvVar)
+	if value == "" {
+		return ctx
+	}
+
+	ctx = useragent.InContext(ctx, upstreamKey, value)
+
+	// Include upstream version as well, if set.
+	value = os.Getenv(upstreamVersionEnvVar)
+	if value == "" {
+		return ctx
+	}
+
+	return useragent.InContext(ctx, upstreamVersionKey, value)
+}

cmd/root/user_agent_upstream_test.go

@@ -0,0 +1,45 @@
+package root
+
+import (
+	"context"
+	"testing"
+
+	"github.com/databricks/databricks-sdk-go/useragent"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestUpstreamSet(t *testing.T) {
+	t.Setenv(upstreamEnvVar, "foobar")
+	ctx := withUpstreamInUserAgent(context.Background())
+	assert.Contains(t, useragent.FromContext(ctx), "upstream/foobar")
+}
+
+func TestUpstreamSetEmpty(t *testing.T) {
+	t.Setenv(upstreamEnvVar, "")
+	ctx := withUpstreamInUserAgent(context.Background())
+	assert.NotContains(t, useragent.FromContext(ctx), "upstream/")
+}
+
+func TestUpstreamVersionSet(t *testing.T) {
+	t.Setenv(upstreamEnvVar, "foobar")
+	t.Setenv(upstreamVersionEnvVar, "0.0.1")
+	ctx := withUpstreamInUserAgent(context.Background())
+	assert.Contains(t, useragent.FromContext(ctx), "upstream/foobar")
+	assert.Contains(t, useragent.FromContext(ctx), "upstream-version/0.0.1")
+}
+
+func TestUpstreamVersionSetEmpty(t *testing.T) {
+	t.Setenv(upstreamEnvVar, "foobar")
+	t.Setenv(upstreamVersionEnvVar, "")
+	ctx := withUpstreamInUserAgent(context.Background())
+	assert.Contains(t, useragent.FromContext(ctx), "upstream/foobar")
+	assert.NotContains(t, useragent.FromContext(ctx), "upstream-version/")
+}
+
+func TestUpstreamVersionSetUpstreamNotSet(t *testing.T) {
+	t.Setenv(upstreamEnvVar, "")
+	t.Setenv(upstreamVersionEnvVar, "0.0.1")
+	ctx := withUpstreamInUserAgent(context.Background())
+	assert.NotContains(t, useragent.FromContext(ctx), "upstream/")
+	assert.NotContains(t, useragent.FromContext(ctx), "upstream-version/")
+}