[rust-axum] don't include quotes in output string from Display trait for string types

[KevinLiddle]

This is sort of a minor breaking change but I think it's still a breaking change, so I put it against 8.0.x. Let me know if you think otherwise.

cc @linxGnu

When generating Rust types for an object defined in the OpenAPI spec as type: string, the generated struct is a 1-tuple with the Display trait using the debug formatting (i.e. "{:?}"). This causes calls to to_string to output the value with quotes in the string.

Example:

components:
  schemas:
    Region:
      description: A geographic region
      type: string

The above schema generates the following type

#[derive(Debug, Clone, PartialEq, PartialOrd, serde::Serialize, serde::Deserialize)]
#[cfg_attr(feature = "conversion", derive(frunk::LabelledGeneric))]
pub struct Region(String);

impl validator::Validate for Region {
    fn validate(&self) -> std::result::Result<(), validator::ValidationErrors> {
        std::result::Result::Ok(())
    }
}

impl std::convert::From<String> for Region {
    fn from(x: String) -> Self {
        Region(x)
    }
}

impl std::fmt::Display for Region {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "{:?}", self.0)
    }
}

impl std::str::FromStr for Region {
    type Err = std::string::ParseError;
    fn from_str(x: &str) -> std::result::Result<Self, Self::Err> {
        std::result::Result::Ok(Region(x.to_string()))
    }
}

impl std::convert::From<Region> for String {
    fn from(x: Region) -> Self {
        x.0
    }
}

impl std::ops::Deref for Region {
    type Target = String;
    fn deref(&self) -> &String {
        &self.0
    }
}

impl std::ops::DerefMut for Region {
    fn deref_mut(&mut self) -> &mut String {
        &mut self.0
    }
}

and if I call to_string on a Region, I will get the value with quotes

let r = Region("the city").to_string() //=> "\"the city\"";

If we change the format string to just "{}", then to_string results in a properly formatted string.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in WSL)
  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
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[wing328]

please target master to start with

we can consider it a bug fix to start with

[KevinLiddle]

@wing328 changed base to master

[linxGnu]

LGTM. Thank you for making the PR

[wing328]

tests passed via #22179