[Feature][Angular]: Represent enum request parameters as enum literal unions

[macjohnny]

Description

It would be nice to have the generated client code contain enums for request parameters that are enum values:

so instead of

public findPetsByStatus(status: Array<string>, observe?: 'body', reportProgress?: boolean): Observable<Array<Pet>>;

you would have

public findPetsByStatus(status: Array<'available' | 'pending' | 'sold'>, observe?: 'body', reportProgress?: boolean): Observable<Array<Pet>>;

which would give a hint to the api user which values are allowed and furthermore allows the compiler to check whether you are calling the api with an invalid parameter.

Swagger-codegen version

2.3.0

Swagger declaration file content or url

swagger petstore example

[macjohnny]

@wing328 can you add this feature to the 2.4.0 milestone?