Add fields 'platforms' and 'status' to Builder.

[MisterOwlPT]

Consider the following example output:

$ docker buildx inspect default
Name:   default
Driver: docker

Nodes:
Name:      default
Endpoint:  default
Status:    running
Platforms: linux/amd64, linux/arm64

I am using python_on_whales on a project and needed to access the values of fields status and platforms.
However, I was only able to access the values of fields name and driver.

With the proposed changes, the following is now possible:

>>> from python_on_whales import docker
>>> builder = docker.buildx.inspect('default')
>>> builder.platforms
['linux/amd64', 'linux/arm64']
>>> builder.status
'running'

In case no platform data is provided the field platforms assumes value [] .

Updated unit tests to consider the proposed changes.

[gabrieldemarmiesse]

Thank you very much! I wish docker buildx had some json formatting options. But until then, splitting it is.

Note that you might need to use some linting for the PR to pass.

docker/buildx#381 was opened and docker/buildx#830 was created to fix it, but it seems it's staled.

[MisterOwlPT]

I run isort and fixed the imports formatting.
PR should pass now.