[REQ] [swift5] async/await `execute` interface of RequestBuilder<T>.

Created by: yosshi4486

Is your feature request related to a problem? Please describe.

A RequestBuilder<T> class already has a closure based execute API, however it doesn't have an async/await based API. Adopting the async interface is more useful in some cases.

Describe the solution you'd like

Add a new async API interface to RequestBuilder<T>, like this.

@discardableResult
func execute() async throws -> Response<T> {

    return try await withTaskCancellationHandler {
        try Task.checkCancellation()
        return try await withCheckedThrowingContinuation { continuation in
            guard !Task.isCancelled else {
              continuation.resume(throwing: CancellationError())
              return
            }

            self.execute { result in
                switch result {
                case let .success(response):
                    continuation.resume(returning: response)
                case let .failure(error):
                    continuation.resume(throwing: error)
                }
            }
        }
    } onCancel: {
        requestTask.cancel()
    }

}

It should be added only when the generator passes async/await option.

Describe alternatives you've considered

None

Additional context

None