[FEATURE] - Handle FirmaTypes as an Enum instead of string

[AnderssonPeter]

Description
This would be a breaking change, but it would be a better DX if FirmaTypes was of type Enum instead of string.

[Johannestegner]

I kind of agree that it would be nice. But as of right now, the other packages makes use of strings, and changing the C# version to use enum would break the "same api in all languages" principle.

I will take it up with the other maintainers and see how they feel.