[Rust] Add support for discriminator

Created by: gferon

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
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Description of the PR

• Add support for the discriminator feature of OpenAPI 3, and implement it with enum in Rust
• Add all missing explicit dyn to trait types to remove warnings
• Add missing re-export for properties that are enum (was missing from #2244).

Example

A schema that looks like:

components:
  schemas:
    Pet:
      type: object
      required:
      - pet_type
      properties:
        pet_type:
          type: string
      discriminator:
        propertyName: pet_type
        mapping:
          cachorro: Dog
          reptilian: Lizard
    Cat:
      allOf:
      - $ref: '#/components/schemas/Pet'
      - type: object
        # all other properties specific to a `Cat`
        properties:
          name:
            type: string
    Dog:
      allOf:
      - $ref: '#/components/schemas/Pet'
      - type: object
        # all other properties specific to a `Dog`
        properties:
          bark:
            type: string
    Lizard:
      allOf:
      - $ref: '#/components/schemas/Pet'
      - type: object
        # all other properties specific to a `Lizard`
        properties:
          lovesRocks:
            type: boolean

would generate a Rust enum like:

#[derive(Debug, PartialEq, Serialize, Deserialize)]
#[serde(tag = "pet_type")]
pub enum Pet {
    #[serde(rename="cachorro")]
    Dog {
        #[serde(rename = "bark", skip_serializing_if = "Option::is_none")]
        bark: Option<String>,
    },
    #[serde(rename="reptilian")]
    Lizard {
        #[serde(rename = "lovesRocks", skip_serializing_if = "Option::is_none")]
        loves_rocks: Option<bool>,
    },
}