Support nested profile_options in profile choices

[rtmiz]

What this adds

profile_options choices can now declare their own profile_options, recursively.
This enables dependent option flows in the spawn form — e.g. a "GPU type" dropdown
that only exists while the "GPU" choice is selected:

c.KubeSpawner.profile_list = [
    {
        "display_name": "Demo",
        "profile_options": {
            "gpu": {
                "display_name": "GPU setup",
                "choices": {
                    "none": {"display_name": "No GPU", "default": True, "kubespawner_override": {}},
                    "nvidia": {
                        "display_name": "NVIDIA",
                        "kubespawner_override": {"extra_resource_limits": {"nvidia.com/gpu": "1"}},
                        "profile_options": {
                            "type": {
                                "display_name": "GPU type",
                                "choices": {
                                    "t4":   {"display_name": "T4", "default": True,
                                             "kubespawner_override": {"node_selector": {"gpu": "t4"}}},
                                    "a100": {"display_name": "A100",
                                             "kubespawner_override": {"node_selector": {"gpu": "a100"}}},
                                },
                            },
                        },
                    },
                },
            },
        },
    },
]

Nesting works to arbitrary depth, and unlisted_choice (including
validation_regex and {value} templating) works at every level.

Native implementation of 2i2c-org/jupyterhub-fancy-profiles#148