Adding callback post create package

[CohenIdo]

Currently, the fleet plugin provides three types of callbacks:

export type ExternalCallback =
  | ExternalCallbackCreate
  | ExternalCallbackDelete
  | ExternalCallbackUpdate;

ExternalCallbackCreate runs before the plugin is installed.
The purpose of this issue is to add a new type of callback that will run after the package policy has been created.

Describe a specific use case for the feature:
The Cloud Security Posture plugin wants to run the callback once the assets for the plugin have been installed.

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[CohenIdo]

WIP

• add new callback type packagePolicyPostCreate
• Add Tests
• types issue in the dynamic function runExternalCallbacks, x-pack/plugins/fleet/server/services/package_policy.ts

PR:

• Adding post create package callback #128744