Enable customisation of the licenses annotation and label

Right now, we hard-code in `docker-bake.hcl` the following annotation:

```
    "index,manifest:org.opencontainers.image.licenses=Apache-2.0",
```

and label:

```
    "org.opencontainers.image.licenses" = "Apache-2.0",
```

While this should be the default, we should enable the redefinition in the metadata on an extension's basis.
