oauth2: Add user-agent member field

Signed-off-by: rja5 <rallen99@gmail.com>

Author: rja5

include/fluent-bit/flb_oauth2.h

@@ -42,6 +42,7 @@ struct flb_oauth2_config {
     flb_sds_t token_url;
     flb_sds_t client_id;
     flb_sds_t client_secret;
+    flb_sds_t user_agent;
     flb_sds_t scope;
     flb_sds_t audience;
     flb_sds_t resource;