Add Base64-encoding/decoding for Google Cloud PubSub plugins (#5543)

plugins/outputs/cloud_pubsub/README.md

@@ -52,6 +52,9 @@ generate it using `telegraf --usage cloud_pubsub`.
   ## Optional. Specifies a timeout for requests to the PubSub API.
   # publish_timeout = "30s"
   
+  ## Optional. If true, published PubSub message data will be base64-encoded.
+  # base64_data = false
+  
   ## Optional. PubSub attributes to add to metrics.
   # [[inputs.pubsub.attributes]]
   #   my_attr = "tag_value"