Feat: add custom retry strategy implementation

Closes immobiliare#79

Documentation/3.Advanced_HTTPClient.md

@@ -77,6 +77,7 @@ The options are:
 - `exponential` and `fibonacci` are the same as `delayed`, but with sequentially increasing delay times
 - `after(HTTPRequest, TimeInterval, AltRequestCatcher?)` will retry the original call after calling an alternate request. For example, if you are making an authenticated request and the session has expired; you can then call a login alternate request to perform a new login and retry the original call.
 - `afterTask(TimeInterval, RetryTask, RetryTaskErrorCatcher?)` performs an async task before retrying the original request. By using an async `Task`, you may perform work outside of the scope of RealHTTP and inject whatever you need into the original request. Note that, like with the existing retry with `HTTPRequest` strategy, any error triggered by the async `Task` is not propagated to the original request. However, a callback could be provided to at least "see" it.
+- `custom` will retry the original call after the amount of seconds returned by the closure you are required to provide for this case. The closure provides you with the failed `HTTPRequest` object so you have the chance to define any custom logic based on it and return the desired amount of seconds.
 
 We'll take a closer look at these strategies below.
 

Sources/RealHTTP/Client/Internal/HTTPResponseValidator/HTTPValidator.swift

@@ -63,13 +63,15 @@ public enum HTTPRetryStrategy {
     public typealias AltRequestCatcher = ((_ request: HTTPRequest, _ response: HTTPResponse) async throws -> Void)
     public typealias RetryTask = ((_ originalRequest: HTTPRequest) async throws -> Void)
     public typealias RetryTaskErrorCatcher = ((_ error: Error) async -> Void)
+    public typealias CustomRetryIntervalProvider = (_ request: HTTPRequest) -> TimeInterval
 
     case immediate
     case delayed(_ interval: TimeInterval)
     case exponential(_ base: Int)
     case fibonacci
     case after(HTTPRequest, TimeInterval, AltRequestCatcher?)
     case afterTask(TimeInterval, RetryTask, RetryTaskErrorCatcher?)
+    case custom(_ retryIntervalProvider: CustomRetryIntervalProvider)
 
     // MARK: - Internal Functions
 
@@ -108,6 +110,9 @@ public enum HTTPRetryStrategy {
 
         case .afterTask:
             return 0
+
+        case .custom(let retryIntervalProvider):
+            return retryIntervalProvider(request)
         }
     }