Remove admin user impersonation from bulk APIs requirements

The App migration platform team provided 2 buk APIs that can only be usedby apps in the migration context:

• Custom fields bulk API: https://jira.atlassian.com/browse/MIG-311
• Entity properties bulk API: https://jira.atlassian.com/browse/MIG-312

These APIs requires apps to impersonate an admin user in order to consume it. That impersonation is not required for single operation APIs. Removing that requirement would greatly ease migration path development.

Workaround

For now, developers can use 

• https://developer.atlassian.com/cloud/jira/platform/rest/v3/api-group-groups/#api-rest-api-3-group-member-get

to retrieve Jira users in an admin group, and then they need to pass the site admin AAID in the header

• Atlassian-Account-Id

to use the Bulk API.