TokenFeature.TokenProvider should be a fun interface

[dalewking]

Providing a TokenFeature.TokenProvider requires creating on object like this:

object : TokenFeature.TokenProvider {
      override fun getToken() = "Bearer ${authentication.accessToken}"`
}

With the Kotlin 1.4 feature of functional interfaces, this interface should be declared as a fun interface allowing to provide a lambda so above code can be replaced with:

{ "Bearer ${authentication.accessToken}" }