Add simple enum support

[clux]

For #39 and #16.

This does not implement sum types properly, it just collects enum members for string constant enums marked as enum. The servicemonitor example now generates more accurate structs. Consider the action field as part of servicemonitor.endpoint.metrics_relabelings:

                          action:
                            default: replace
                            description: Action to perform based on regex matching.
                              Default is 'replace'
                            enum:
                            - replace
                            - keep
                            - drop
                            - hashmod
                            - labelmap
                            - labeldrop
                            - labelkeep
                            type: string

produces:

/// RelabelConfig allows dynamic rewriting of the label set, being applied to samples before ingestion. It defines `<metric_relabel_configs>`-section of Prometheus configuration. More info: https://prometheus.io/docs/prometheus/latest/configuration/configuration/#metric_relabel_configs
#[derive(Serialize, Deserialize, Clone, Debug)]
pub enum ServiceMonitorEndpointsMetricRelabelingsAction {
    replace,
    keep,
    drop,
    hashmod,
    labelmap,
    labeldrop,
    labelkeep,
}

Naming consistency with rust will need to be addressed in a later PR.