API to get current user for Account Client

[antsok]

Problem Statement
Given that there is an account-level identity (user or a service principal) with account admin permissions that is not granted access to any workspace,
When this identity is used by a pipeline,
We need to know the id of this identity (using the account client API), so that this id can be used in further API calls.

Currently, CurrentUserAPI is only available for workspace client, so there seems to be no way of finding out Databricks identity Id. We have to hardcode it.

Proposed Solution
Make something like CurrentUserAPI also available in account client, and make it work for users and service principals.

Additional Context
Add any other context, references or screenshots about the feature request here.