Add support for IAM tokens (#672)

* add support for iam tokens

* revert example

* add credentials check

* manage iam tokens

* add tests for token logic in base service

* make the token manager its own service

* remove old token request handler

* use sendRequest in token manager

* add tests

* use camelCase

* add base service tests

* make request/refresh methods private

* use iamUrl

* check for null token manager in setAccessToken

Author: dpopp07

.gitignore

@@ -29,6 +29,7 @@ text-to-speech/*.js
 speech-to-text/*.js
 authorization/*.js
 assistant/*.js
+iam-token-manager/*.js
 index.js
 .nyc_output
 **/*.d.ts