Kibana version:
7.8.0
Elasticsearch version:
7.8.0
Server OS version:
Cloud ESS
Browser version:
Any Browser
Browser OS version:
Any OS

Describe the bug:
Added HTTP header to Webhook connector is not persisted after saving the webhook connector.

Steps to reproduce:
1.Kibana > Stack Management > Alerts and Actions > Connectors > Create Webhook Connector > Add HTTP Header > Save
2. Edit newly created connector > Added HTTP Header not listed.

Expected behavior:
Editing the connector should show the added HTTP headers.

Errors in browser console (if relevant):
None

Any additional context:
I've been unable to check if the header is persisted somewhere else, but this is because I don't know where to look.