Support token providers in Bearer Auth middleware.

Sources/ASConnectService/Catalogs/ASConnectService.docc/Library.md

@@ -25,6 +25,14 @@ let client = Client(
 )
 ```
 
+Tokens expire after 20 minutes at most. For longer-lived clients, pass a token provider instead, which is called on every request:
+
+```swift
+BearerAuthMiddleware {
+    try await tokenStore.validToken()
+}
+```
+
 ### Making API Calls
 
 The ``Client`` conforms to ``APIProtocol``, which defines a method for every endpoint in the App Store Connect API. Each method accepts an `Input` value and returns an `Output` value with the response.