Add proguard config for R8 to keep type information for generic signatures of retrofit2.Response

[ottne]

This rule is needed with R8 full mode for interfaces such as in the example below, as R8 strips generic signatures of Response.

Follow up of #3596.

interface WebApi {
    @GET("/api/data")
    fun getData(): Single<Response<MyData>>

    @GET("/api/data")
    suspend fun getDataSuspending(): Response<MyData>
}