Add convenience methods for sending Client requests with a Content body

[owainhunt]

Adds post, put and patch convenience functions for sending Client requests with a Content body.

E.g.

struct MyData: Content {
  let title: String
  let count: Int
}

let myData = MyData(...)

let response = try await req.client.post(url, myData)

[0xTim]

Thanks!