Add ChangeOwnPassword method for self-service password changes

[jinapurapu]

Summary

• Add ChangeOwnPassword method to allow users to change their own password
• This endpoint bypasses IAM policy checks, enabling password changes even with explicit deny on
  admin:CreateUser

Motivation

Previously, changing a user's password required the admin:CreateUser permission via
SetUser/AddUser. This made it impossible for users to change their own password if they had an
explicit deny on that action. The new dedicated endpoint allows self-service password changes without
elevated permissions.

[harshavardhana]

ChangeMyPassword is perhaps a better API name.