feat(openapi): reflect fallback security scheme in spec

[patrickariel]

This makes it so that security schemes with a fallback:

#[derive(SecurityScheme)]
enum MySecurityScheme {
    MySecuritySchemeBasic(MySecuritySchemeBasic),
    #[oai(fallback)]
    NoAuth,
}

Will be reflected in the OpenAPI schema:

"security": [
  {
    "MySecuritySchemeBasic": []
  },
  {} // <-- the fallback scheme
]

This is the correct way to define optional security schemes, according to the official specification as of version 3.0.3.

Closes #959

[stevenj]

This looks great. Would love to see this merged.

[attila-lin]

Thank you