[Watcher] Re-enter password for webhook action on edit

[jloleysens]

Kibana version:

Testing on 7.7 13066a48d5, but should be present on master too.

Describe the bug:

When a watch has been created with a webhook action editing that watch results in the "password" field of the watch being cleared. If you wanted to edit something else, like the watcher schedule, you will need to re-enter the password for the webhook integration too otherwise you cannot save (it is a required field).

Steps to reproduce:

1. Create a threshold watch with a webhook action
2. Save the watch
3. Attempt to edit the schedule of the watch that was just created

Expected behavior:

The watch should not require re-entering the password field and it should not display any value in the password field. It should only require entering the password when I first created the webhook action.

Screenshots (if relevant):

[elasticmachine]

Pinging @elastic/es-ui (Team:Elasticsearch UI)

[alisonelizabeth]

Related issue: #21010