Added an internal API for overriding the API base URL (#5739)

* Added an internal API for overriding the API base URL

* Updated and added tests for API baseURL configuration option

* Revert change that removed serverHostURL from HTTPRequestPath since it broke other types implementing this protocol

* swiftlint

* Removed the apiBaseURL property from Purchases since it's only used for internal testing anyways

* Remove unused @_spi(Internal)

* Updated comment on api base URL vs proxy URL

* Fixed linting issue

Sources/Networking/HTTPClient/HTTPRequestPath.swift

@@ -123,8 +123,9 @@ extension HTTPRequest {
 
 extension HTTPRequest.Path: HTTPRequestPath {
 
-    // swiftlint:disable:next force_unwrapping
-    static let serverHostURL = URL(string: "https://api.revenuecat.com")!
+    static var serverHostURL: URL {
+        SystemInfo.apiBaseURL
+    }
 
     private static let fallbackServerHostURL = URL(string: "https://api-production.8-lives-cat.io")