[rust][client] fix breaking regression introduced by #10432

[Foorack]

#10432

This change forgot enum structures, which causes the compiler to throw errors for "the trait Default is not implemented for MyEnum". This change implements the Default trait to the enum template.

This has been tested to work and fixes the problem.

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@frol (2017/07) @farcaller (2017/08) @richardwhiuk (2019/07) @paladinzh (2020/05)

[xli]

+1 facing the issue.

[wing328]

Tested locally with updated samples and the result is good:

   Compiling reqwest v0.9.24
   Compiling petstore-reqwest v1.0.0 (/Users/williamcheng/Code/openapi-generator/samples/client/petstore/rust/reqwest/petstore)
    Finished test [unoptimized + debuginfo] target(s) in 1m 27s
     Running unittests (/Users/williamcheng/Code/openapi-generator/samples/client/petstore/rust/target/debug/deps/petstore_reqwest-64fb8d120c068931)

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

     Running tests/pet_tests.rs (/Users/williamcheng/Code/openapi-generator/samples/client/petstore/rust/target/debug/deps/pet_tests-a306d64f90bd2c91)

running 1 test
test test_pet ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.03s

   Doc-tests petstore-reqwest

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

Thanks for the fix.

[wing328]

@Foorack when you've time, I wonder if you can contribute a test under samples/client/petstore/rust/reqwest/ to help catch the issue moving forward. Thanks in advance.

[aeneasr]

This introduced another regression that can happen:

error[E0277]: the trait bound `UiNodeAttributes: Default` is not satisfied
   --> src/models/ui_node.rs:18:5
    |
15  | #[derive(Clone, Debug, PartialEq, Default, Serialize, Deserialize)]
    |                                   ------- in this derive macro expansion
...
18  |     pub attributes: Box<crate::models::UiNodeAttributes>,
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Default` is not implemented for `UiNodeAttributes`
    |
    = note: required because of the requirements on the impl of `Default` for `Box<UiNodeAttributes>`
note: required by `std::default::Default::default`
    = note: this error originates in the derive macro `Default` (in Nightly builds, run with -Z macro-backtrace for more info)

See also: #10845