[Rust Server] Handle array of objects inside an object correctly

[richardwhiuk]

For objects, which contain arrays of wrapped strings - e.g.

    ObjectWithArrayOfObjects:
      type: object
      properties:
       objectArray:
         type: array
         items:
           $ref: '#/components/schemas/StringObject'
    StringObject:
      type: string

This currently produces:

struct ObjectWithArrayOfObjects {
    objectArray: Option<Vec<String>>
}

struct StringObject(String);

whereas we should be doing:

struct ObjectWithArrayOfObjects {
    objectArray: Option<Vec<StringObject>>
}

struct StringObject(String);

We actually get it right in one place (when converting header parameters), and wrong in the model definition, which breaks some APIs.

Commits are broken down into:

• Fixing the bug
• Adding a test case
• Updating the samples

Contribution by myself on behalf of @Metaswitch. Reviewed by @mirw

Rust Technical Committee

@frol @farcaller @bjgill

PR checklist

• Read the contribution guidelines.
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