[Agentless] Expose is_default property on deployment_modes items

[kpollich]

Depends on elastic/package-spec#850

In elastic/package-spec#850 we're adding support for a new is_default property on items in the deployment_modes array for integration manifests. This flag allows integration maintainers to have a granular control of which deployment mode option is selected by default when configuring an agentless integration. We need to expose this new property in the package info API.