Document available handlebars helpers in `agent/spec.yml` file

[kpollich]

As called out in https://github.com/elastic/kibana/pull/135992#issuecomment-1181825725, we should include documentation for any available templating helpers in the agent/spec.yml file for each package type.

The available helpers today are

• contains - Utility for checking whether a list or string contains an item
• escape_string - Wraps a string in single quotes + escapes nested single quotes and newlines
• to_json - Runs a given value through JSON.stringify and returns the resulting string

See https://github.com/elastic/kibana/blob/main/x-pack/plugins/fleet/server/services/epm/agent/agent.ts for the code where Fleet registers these helpers in its templating engine.

These spec docs should include a written description of each helper (see above) as well as example usage.

[ruflin]

++ on adding these docs. Lets make sure we find a way that these docs are easy accessible to every engineer building integrations without having to read docs inside a yaml file. Of course, the spec file should be source of truth.