chore(protocol-http): guidance for HttpRequest cloning

[kuhe]

This moves HttpRequest#clone() (instance method) to HttpRequest.clone(IHttpRequest) (static method) and deprecates the original instance method.

HttpRequest#clone() cannot safely be called, because the type-guard HttpRequest.isInstance(unknown) is not correctly implemented, but cannot safely be changed since it is widely used as-is.

Context:

HttpRequest.isInstance(unknown) -> boolean is HttpRequest is actually
HttpRequest.isInstance(unknown) -> boolean is IHttpRequest.

It returns true when the input satisfies the data/structural interface, but the structural interface does not necessarily include the .clone() method.

[kuhe]

In a later major version of @smithy/protocol-http, the HttpRequest.isInstance(unknown) function can be made less misleading with a non-backwards-compatible change.