Add 'update' methods, which allow for repeated calls to Requests.

[ezraberch]

This PR adds functionality to Request which allows for the Request to be repeated after the initial call. An example use case is an automatically-updating RequestView.

The following SwiftUI view is an example of this. The RequestView will call the request to load the content once when the view loads. Thereafter, the view will automatically update every 10 seconds. The view will also be updated whenever the user taps the button.

let subject = PassthroughSubject<Void, Never>()

    struct ContentView: View {
        var body: some View {
            VStack {
                Button(action: {
                    subject.send()
                }, label: {
                    Text("Refresh")
                })
                RequestView (Request {
                    Url("https://worldtimeapi.org/api/ip.txt")
                }.update(publisher: subject).update(every: 10)) {data in
                    Text(String(decoding: data ?? Data(), as: UTF8.self))
                    Text("placeholder")
                }
            }
        }
    }

[codecov-commenter]

Codecov Report

Merging #24 into master will increase coverage by 0.25%. The diff coverage is 92.50%.

@@            Coverage Diff             @@
##           master      #24      +/-   ##
==========================================
+ Coverage   85.77%   86.03%   +0.25%     
==========================================
  Files          21       21              
  Lines         682      716      +34     
==========================================
+ Hits          585      616      +31     
- Misses         97      100       +3     

Impacted Files	Coverage Δ
Sources/Request/Request/Request.swift	93.26% <89.28%> (-1.86%)	:arrow_down:
Tests/RequestTests/RequestTests.swift	93.28% <100.00%> (+0.33%)	:arrow_up:

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[carson-katri]

This looks great!

I have two notes:

1. Lets make public func update(every seconds: Double) -> Self use TimeInterval instead of Double.
2. Does it need combineIdentifier if the class doesn't conform to CustomCombineIdentifierConvertible? Also, could it be a constant instead of a variable?

[ezraberch]

1. Done.
2. The class does conform to CustomCombineIdentifierConvertible as Subscriber inherits from it. Also note from https://developer.apple.com/documentation/combine/customcombineidentifierconvertible:

If you create a custom Subscription or Subscriber type, implement this protocol so that development tools can uniquely identify publisher chains in your app. If your type is a class, Combine provides an implementation of combineIdentifier for you. If your type is a structure, set up the identifier as follows: let combineIdentifier = CombineIdentifier()

You're right that it should be a constant; that has been fixed.