feat: add support for custom context to task-context

[bhearsum]

This is somewhat of a continuation to #968 in that it would allow for more resolve_keyed_by calls to be replaced by task-context. As a concrete example, we use by-release-level in many places in gecko. Its value is derived from parameters and some constants. With this patch, we could register it as custom context and replace a whole bunch of resolve_keyed_by calls in transforms, eg:

In some python code that's called at taskgraph registration:

@custom_context("release-level")
def release_level_context(config, task):
    """Provide ``release-level`` ("production" or "staging"), derived from
    parameters, so ``by-release-level`` fields can be resolved declaratively."""
    return {"release-level": release_level(config.params)}

And in various kinds we'd end up with things like:

transforms:
    - taskgraph.transforms.task_context

task-defaults:
    task-context:
        from-custom:
            - release-level
        substitution-fields:
            - scopes

    scopes:
        by-release-level:
            production:
                - project:releng:ship-it:server:production
                - project:releng:ship-it:action:create-new-release
            staging:
                - project:releng:ship-it:server:staging
                - project:releng:ship-it:action:create-new-release

[jcristau]

Consider adding to task_context transform docs?

[bhearsum]

Consider adding to task_context transform docs?

er, yes! I meant to do that before moving it out of draft.