Added an API key token provider.

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

@@ -23,11 +23,17 @@ 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:
+Tokens expire after 20 minutes at most. For longer-lived clients, use ``APIKeyTokenProvider``, which signs new ones with your API key as needed:
 
 ```swift
+let tokenProvider = try APIKeyTokenProvider(
+    issuerID: yourIssuerID,
+    keyID: yourKeyID,
+    privateKey: yourP8FileContents
+)
+
 BearerAuthMiddleware {
-    try await tokenStore.validToken()
+    try await tokenProvider.token()
 }
 ```
 
@@ -67,6 +73,7 @@ repeat {
 ### Authentication
 
 - ``BearerAuthMiddleware``
+- ``APIKeyTokenProvider``
 
 ### Generated Types