Generate Refit interfaces as partial by default

[kirides]

I propose ~~ a --partial Flag for the CLI.~~ refit interfaces to be generated as partial interfaces by default, which would comply with most other code-generator tools out there and the way the models are generated currently.

Reasoning being, for certain classes we need to add attributes, interfaces or utility methods which is currently not possible without modifying the generated code.

By having partial classes for Models and Refit-Interfaces, we can make use of a generated.additions.cs (example name) which contains the needed modifications.

// a.cs
internal partial interface ISomething
{

}

// a.additions.cs
[FancyAttribute("With Value")]
[Headers("X-App: MyApp")]
partial interface ISomething {}

[christianhelle]

@kirides The models are already partial.

How about we make the interfaces partial by default as well? I can't really see any reason for not making it partial to begin with

[kirides]

Sounds great @christianhelle

totally forgot that Models are partial by default, even though we already use them this way.

I updated the title and description to reflect the spec change :)