Add environments REST API

[davidbrochart]

After #534 which allowed to run (all) kernels in a separate environment than the environment of the server, this PR allows to:

• create environments at POST /api/environments, which returns an environmend_id. The name of the package manager and a path to an environment file must be passed in the body.
• wait for an environment creation at GET /api/environments/wait/{environment_id}.
• poll for an environment status at GET /api/environments/status/{environment_id}, which returns e.g. "environment creation success".
• launch a kernel in a specific environment at POST /api/sessions with a body like so: {..., "kernel": {"name": "python3", "environment_id": "012-abc"}}

[trungleduc]

I have some questions:

create environments at POST /api/environments, which returns an environmend_id.

The env id is returned immediately without waiting for the Mamba command to finish?

launch a kernel in a specific environment at POST /api/sessions with a body like so: {..., "kernel": {"name": "python3", "environment_id": "012-abc"}}

What is the behavior if the environment_id is missing?

[davidbrochart]

create environments at POST /api/environments, which returns an environmend_id.

The env id is returned immediately without waiting for the Mamba command to finish?

Yes.

launch a kernel in a specific environment at POST /api/sessions with a body like so: {..., "kernel": {"name": "python3", "environment_id": "012-abc"}}

What is the behavior if the environment_id is missing?

The kernel is launched in the server environment.