datastore: allow simple enums as map keys

[webern]

Issue number:

Closes #2686

Description of changes:

This allows us to use an enum as a map key in the model, which is something we want to do in the upcoming implementation for #1703

Testing done:

Created this enum

#[derive(Debug, Clone, Eq, PartialEq, Hash, Serialize, Deserialize, Scalar)]
#[serde(rename_all = "snake_case")]
pub enum RandomWords {
    Gloom,
    TheftClash,
}

Added it to the EcsSettings struct

// ECS settings.
#[model]
struct ECSSettings {
    /* ... */
    stuff: HashMap<RandomWords, String>,
}

Ran an aws-ecs-1 variant:

Tried to set a bad map key, api rejected it as expected:

apiclient set ecs.stuff.bad_key=value
Unable to match your input to the data model.  We may not have enough type information.  Please try the --json input form.  Cause: Error during deserialization: unknown variant `bad_key`, expected one of `begin`, `bet`, `crack`, `elect`, `fix`, `gloom`, `item`, `lump`, `new`, `news`, `strap`, `theft_clash`, `time`, `tip`

Set a couple of good key/vals:

apiclient set ecs.stuff.theft_clash=value_1
apiclient set ecs.stuff.gloom=value_2

Get the map:

apiclient get settings.ecs.stuff
{
  "settings": {
    "ecs": {
      "stuff": {
        "gloom": "value_2",
        "theft_clash": "value_1"
      }
    }
  }
}

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[yeazelm]

LGTM

[stmcginnis]

Looks good!