First-class Swift concurrency support

[mkj-is]

Are you still using/maintaining the project?

If yes, now it is as good time as ever to change building requests to Swift concurrency.

https://github.com/futuredapp/FTAPIKit/blob/main/Sources/FTAPIKit/Server.swift#L29

The buildRequest method is not async, which makes any request signing and request creation unnecessarily complex. This change would allow some internal improvements to the whole framework, which could be relased as a new major version.