429 Too Many Requests

Problem/Motivation

We have a lot of custom Sendethic attributes and it seems the module gets them one by one and hits the API limit:

Client error: `GET https://services.message-business.com/api/rest/v4/account/contactattribu...` resulted in a `429 Too Many Requests` response:
"API calls quota exceeded! maximum admitted 40 per Second. Please retry later or contact us for more informations."

Steps to reproduce

Have dozens of custom attributes.
Browse to /admin/config/system/sendethic or /admin/structure/webform/manage/[webform_id]/handlers/sendethic/edit

Proposed resolution

Maybe the module could wait a second after getting 30 attributes to be sure to not hit the limit.

Remaining tasks

User interface changes

API changes

Data model changes