cli/command: add WithUserAgent option

Add support to the `cli/command` package to accept a custom User
Agent to pass to the underlying client.

This is used as the `UpstreamClient` portion of the `User-Agent`
when the Moby daemon makes requests.

For example, pushing and pulling images with Compose might result
in the registry seeing a `User-Agent` value of:

```
docker/24.0.7 go/go1.20.10 git-commit/311b9ff kernel/6.5.13-linuxkit os/linux arch/arm64 UpstreamClient(docker-cli-plugin-compose/v2.24.0)
```

Signed-off-by: Milas Bowman <milas.bowman@docker.com>
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/cli_options.go

@@ -3,6 +3,7 @@ package command
 import (
 	"context"
 	"encoding/csv"
+	"errors"
 	"fmt"
 	"io"
 	"net/http"
@@ -236,3 +237,14 @@ func withCustomHeadersFromEnv() (client.Opt, error) {
 	//  see https://github.com/docker/cli/pull/5098#issuecomment-2147403871  (when updating, also update the WARNING in the function and env-var GoDoc)
 	return client.WithHTTPHeaders(env), nil
 }
+
+// WithUserAgent configures the User-Agent string for cli HTTP requests.
+func WithUserAgent(userAgent string) CLIOption {
+	return func(cli *DockerCli) error {
+		if userAgent == "" {
+			return errors.New("user agent cannot be blank")
+		}
+		cli.userAgent = userAgent
+		return nil
+	}
+}