[FEATURE] Separate Api classes for each Controller [Api]

[xeinebiu]

Take as example the following library
https://www.npmjs.com/package/openapi-typescript-codegen

It is able to generate something like

final authApi = AuthApi.create(...);  // auth as api tag
final postsApi = PostsApi.create(...); // posts as api tag

authApi.signIn(...);  // signIn is the operationId
postsApi.getAll(...); // getAll is the operationId

while, the swagger-dart-code-generator is only able to provide something like

final demoApi = DemoApi.create(...);

demoApi.apiAuthPost(...);
demoApi.apiPostsGetAllPost(...);

We can see some issues like operationId is not respected. each method has some generated prefix and postfix. Swagger tags are not used to generate separate API classes like openapi-typescript-codegen does.