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>
048e931b42