Support both mapped and typed input schemas in Fleet integration polies

[tobio]

Related to https://github.com/elastic/terraform-provider-elasticstack/issues/601

Integration policies support two body formats, a map ({"input-id": {...}}) and a list ([{"type":"endpoint"...}]). Only the latter form supports the config field used by both Endpoint and APM integrations (even though that form is apparently deprecated 🤷).

Functionally, this PR should be a no-op. It adds support for the latter input form in the API client, and updates the integration policy resource to match the new generated API client. It doesn't make any resource schema changes, and the behaviour should be identical.