[Rust Server] Support parameters correctly in response headers

Created by: richardwhiuk

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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  • @frol, @farcaller, @bjgill

Description of the PR

Key change is in modules/openapi-generator/src/main/resources/rust-server/lib.mustache

The code currently only emits an opening bracket (and body) for the structured enum response entry if:

• the response contains headers
• this is the first response

This means we end up with nonsense code like:

#[derive(Debug, PartialEq)]
pub enum CreateResponse {
    /// OK
    Ok { body: String, e_tag: String } ,
    /// Error response
    ErrorResponse , reason: String } ,
}

which obviously doesn't compile.

I've restructured the mustache template so that it's a lot clearer about how it's built up. This exploits the templating library, which will remove a line entirely if there's only a single if / endif tag on it - see https://github.com/samskivert/jmustache#newline-trimming for details of that.

A basic test has been added to modules/openapi-generator/src/test/resources/3_0/rust-server/openapi-v3.yaml