Allow single strings in watcher emails

[pquentin]

While the values are stored as arrays, single strings are allowed in the parser: https://github.com/elastic/elasticsearch/blob/32eada688f7de63c75a22c4b9f6464aab75c01b7/x-pack/plugin/watcher/src/main/java/org/elasticsearch/xpack/watcher/notification/email/EmailTemplate.java#L398-L441.

[github-actions]

Following you can find the validation results for the APIs you have changed.

API	Status	Request	Response
watcher.ack_watch	🟢	1/1	1/1
watcher.activate_watch	🟢	1/1	1/1
watcher.deactivate_watch	🟢	1/1	1/1
watcher.delete_watch	🟢	2/2	2/2
watcher.execute_watch	🟢	7/7	7/7
watcher.get_settings	⚪	Missing test	Missing test
watcher.get_watch	🟢	9/9	9/9
watcher.put_watch	🔴	29/38	38/38
watcher.query_watches	⚪	Missing test	Missing test
watcher.start	🟢	1/1	1/1
watcher.stats	🟢	1/1	1/1
watcher.stop	🟢	1/1	1/1
watcher.update_settings	⚪	Missing test	Missing test

You can validate these APIs yourself by using the make validate target.