Improve extensiblity for SwaggerGenerator

[KingJiangNet]

Expose ApiExplorer to make SwaggerGenerator more extensible, rather than using other heavy tools such as NSwag.

[domaindrivendev]

I don't believe this PR is necessary. You can already access ApiExplorer from the HttpConfiguration object:

var apiExplorer = httpConfig.Services.GetApiExplorer();

[KingJiangNet]

@domaindrivendev ,
The purpose of exposing ApiExplorer is for replacing the default api explorer, so that we can have plenty of possibilities to extend this tool, such as loading api from external assemblies, and even encapsulate some non-WebApi to standard WebApi through some dynamic design. If there is no such dynamic support, its scope is limited too much. Actually NSwag did some(it claims it is a super set of Smashbukke) , but it seems too heavy and not that flexible.

[domaindrivendev]

Thx for the explanation. You can already do that too. Use httpConfig.Services.Replace

[KingJiangNet]

Yes, it is a way.