[Watcher] Enable partial updates to a watch

[jloleysens]

Describe the feature:

The _watcher API provides a mechanism for creating and updating watches (https://www.elastic.co/guide/en/elasticsearch/reference/7.7/watcher-api.html). After a watch has been created this endpoint can be used to update the watch in its entirety.

However, if we have watch with a webhook action that contains a redacted password field:

    "actions": {
      "webhook_1": {
        "webhook": {
          "scheme": "http",
          "host": "localhost",
          "port": 8000,
          "method": "get",
          "path": "fgvgh23",
          "params": {},
          "headers": {},
          "auth": {
            "basic": {
              "username": "test",
              "password": "::es_redacted::"
            }
          },
          "body": "{\n  \"message\": \"Watch has exceeded the threshold\"\n}"
        }
      }
    },

users must re-enter the password even if they only want to update the watch schedule (something unrelated to actions), see the effect of this in Kibana here: elastic/kibana#61781.

It would be great if there were a way to update a watch in a more piecemeal fashion - a way to do partial updates.

[elasticmachine]

Pinging @elastic/es-core-features (:Core/Features/Watcher)

[elasticmachine]

Pinging @elastic/es-ui (:ES-UI)