<!--- Verify first that your feature was not already discussed on GitHub -->
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##### SUMMARY
<!--- Describe the new feature/improvement briefly below -->
New Percona/MySQL 8 privileges with underscore is not working. 
the following error is coming 

`MSG: invalid privileges string: Invalid privileges specified: frozenset({'BACKUP_ADMIN'})`

##### ISSUE TYPE
- Feature Idea

##### COMPONENT NAME
<!--- Write the short name of the module, plugin, task or feature below, use your best guess if unsure -->
mysql_user module
most likely the new privileges needs to be added to lib/ansible/modules/database/mysql/mysql_user.py

##### ADDITIONAL INFORMATION
<!--- Describe how the feature would be used, why it is needed and what it would solve -->

<!--- Paste example playbooks or commands between quotes below -->
```yaml

```

<!--- HINT: You can also paste gist.github.com links for larger files -->

version_added: '2.10'
  plugin_hash_string:
    description:
      - User's plugin hash string (``CREATE USER user IDENTIFIED WITH plugin AS plugin_hash_string``).
    type: str
    version_added: '2.10'
  plugin_auth_string:
    description:
      - User's plugin auth_string (``CREATE USER user IDENTIFIED WITH plugin BY plugin_auth_string``).
    type: str
    version_added: '2.10'
notes:
   - "MySQL server installs with default login_user of 'root' and no password. To secure this user
     as part of an idempotent playbook, you must create at least two tasks: the first must change the root user's password,
     without providing any login_user/login_password details. The second must drop a ~/.my.cnf file containing
     the new root credentials. Subsequent runs of the playbook will then succeed by reading the new credentials from
     the file."
   - Currently, there is only support for the `mysql_native_password` encrypted password hash module.
seealso:
- module: mysql_info
- name: MySQL access control and account management reference
  description: Complete reference of the MySQL access control and account management documentation.
  link: https://dev.mysql.com/doc/refman/8.0/en/access-control.html
author:
- Jonathan Mainguy (@Jmainguy)
- Benjamin Malynovytch (@bmalynovytch)
- Lukasz Tomaszkiewicz (@tomaszkiewicz)
extends_documentation_fragment: mysql
'''
EXAMPLES = r'''