[Core, Rust Server] Support optional headers

[richardwhiuk]

Rust Server currently treats all headers declared in the OpenAPI file as mandatory, when actually they default to optional.

Furthermore, the core code doesn't pass through whether a header is mandatory or optional.

This makes a small change to the DefaultCodgen to pass this data through, defaulting correctly.

The Rust change is then fairly straightforward - we enhance Rust Server, using Option to represent non-required headers.

This is breaking, because we'll treat headers which were treated as required before as optional, and we'll provide them as an Option instead.

Rust Server Technical Committee

• @frol
• @farcaller
• @bjgill
• @paladinzh

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• File the PR against the correct branch: master, 4.3.x, 5.0.x. Default: master.
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

[richardwhiuk]

@OpenAPITools/generator-core-team - Minor core change as part of this.