Improve error reporting when a union is discriminated by a Literal type field

[NiklasRosenstein]

Example error message:

databind.core.converter.ConversionError: unable to deserialize any union member

Trace:
    $: TypeHint(helsing.scm.resources.v1alpha1.TerraformWorkspace.TerraformWorkspace)
    .workflow: TypeHint(typing.Union[helsing.scm.resources.v1alpha1.TerraformWorkspace.TerraformWorkspace.ApiDrivenWorkflow, helsing.scm.resources.v1alpha1.TerraformWorkspace.TerraformWorkspace.CliDrivenWorkflow, helsing.scm.resources.v1alpha1.TerraformWorkspace.TerraformWorkspace.VcsDrivenWorkflow])

The following errors have been reported by converters:

  databind.json.converters.SchemaConverter(): encountered extra keys: {'providersFrom'}

    Trace:
        $: TypeHint(helsing.scm.resources.v1alpha1.TerraformWorkspace.TerraformWorkspace)
        .workflow: TypeHint(typing.Union[helsing.scm.resources.v1alpha1.TerraformWorkspace.TerraformWorkspace.ApiDrivenWorkflow, helsing.scm.resources.v1alpha1.TerraformWorkspace.TerraformWorkspace.CliDrivenWorkflow, helsing.scm.resources.v1alpha1.TerraformWorkspace.TerraformWorkspace.VcsDrivenWorkflow])
        ^: TypeHint(helsing.scm.resources.v1alpha1.TerraformWorkspace.TerraformWorkspace.ApiDrivenWorkflow)

  databind.json.converters.LiteralConverter(): literal value mismatch: got 'ApiDriven', expected 'CliDriven'

    Trace:
        $: TypeHint(helsing.scm.resources.v1alpha1.TerraformWorkspace.TerraformWorkspace)
        .workflow: TypeHint(typing.Union[helsing.scm.resources.v1alpha1.TerraformWorkspace.TerraformWorkspace.ApiDrivenWorkflow, helsing.scm.resources.v1alpha1.TerraformWorkspace.TerraformWorkspace.CliDrivenWorkflow, helsing.scm.resources.v1alpha1.TerraformWorkspace.TerraformWorkspace.VcsDrivenWorkflow])
        ^: TypeHint(helsing.scm.resources.v1alpha1.TerraformWorkspace.TerraformWorkspace.CliDrivenWorkflow)
        .type: TypeHint(typing.Literal['CliDriven'])

  databind.json.converters.SchemaConverter(): missing required field: 'vcsProvider'

    Trace:
        $: TypeHint(helsing.scm.resources.v1alpha1.TerraformWorkspace.TerraformWorkspace)
        .workflow: TypeHint(typing.Union[helsing.scm.resources.v1alpha1.TerraformWorkspace.TerraformWorkspace.ApiDrivenWorkflow, helsing.scm.resources.v1alpha1.TerraformWorkspace.TerraformWorkspace.CliDrivenWorkflow, helsing.scm.resources.v1alpha1.TerraformWorkspace.TerraformWorkspace.VcsDrivenWorkflow])
        ^: TypeHint(helsing.scm.resources.v1alpha1.TerraformWorkspace.TerraformWorkspace.VcsDrivenWorkflow)

The value of the discriminator is set to VcsDriven, then only the matching members of the union should be tried.