Marker interface for swagger ui index transformer

[natrem]

close #745

as explained here it eases providing a custom SwaggerIndexTransformer by introducing an intermediary interface.

@bnasslahsen please note the 2nd commit. I factorized some behavior in the abstract class. The rationale is that if you have to duplicate this in mvc and reactive implementations then application creating a new transformation (for instance the linked example) will face the same issue and may have to rewrite same code.
If you're not comfortable with it I don't mind dropping this part; since you have overwriteSwaggerDefaultUrl and addInitOauth I supposed we can add other methods related to the default implementation.

Since you rely on integration tests I did not add specific unit tests on the autoconfiguration but I can if you wish (I've done a few lately so it won't take long)

[bnasslahsen]

Thank you @natrem for your contribution 👍
I couldn't resist to renaming the classes SwaggerIndexPageTransformer and calling the interface SwaggerIndexTransformer. which makes code much cleaner and less impacted files.

It will be available for the next release v1.4.3.

[natrem]

I agree the renaming is better but I didn't dare.
thanks again for your job in maintaining these useful libraries