refactor: remove SignableRequest

[flaneur2020]

          Thanks a lot for your PR! However I'm thinking of a big refactor that build a `Sign` for entire reqsign. And we can remove the `SignableRequest` concept entirely by just support `http`. Do you have interest in this?

Originally posted by @Xuanwo in #459 (comment)

[flaneur2020]

@Xuanwo can you help assign this issue to me?

[Xuanwo]

Great, have fun!