Expose allowed values for app settings exposed as enum types

[nblumhardt]

See datalust/seq-tickets#1029

Allow Seq apps to use enum types for settings, which the Seq UI can expose as a select list.

enum GelfListenerProtocol
{
    [Display("TCP")]
    Tcp,

    [Display("UDP")]
    Udp
}

// Elsewhere...
[SeqAppSetting]
public GelfListenerProtocol Protocol { get; set; }

The two components of the change are:

1. inputType: Select and allowedValues in the definition format:

    "settings": {
      "protocol": {
        "displayName": "GELF address",
        "inputType": "Select",
        "allowedValues": [
            {"value": "Tcp", "description": "TCP"},
            {"value": "Udp", "description": "UDP"}
        ]
      },

and 2., support for conversion from string values such as "Tcp" back to enum members when initializing the app object.

The change is implemented in such a way that the Seq.Apps package doesn't immediately need an update: app authors opt in by using enum types for app settings. Seq versions prior to 2020.5 won't be able to load these, however, so widely-used public apps will only be realistically able to depend on this after a transition period (perhaps, 6-12 months).