[Fleet] Don't add extra quotes to YAML strings from manifest files (#…

…93585) (#93778)

* Add UI validation for string YAML values in policies.

* Do not quote YAML strings containing special characters.

* Add test case for wildcards in the middle of strings.

* Add multiline test case.

* Polish test case.

* Update API docs

Co-authored-by: Kibana Machine <[email protected]>
# Conflicts:
#	api_docs/fleet.json