<!--- Verify first that your feature was not already discussed on GitHub -->
<!--- Complete *all* sections as described, this form is processed automatically -->

##### 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 -->

state: present
- name: Revoke all privileges for user 'bob' and password '12345'
  mysql_user:
    name: bob
    password: 12345
    priv: "*.*:USAGE"
    state: present
# Example privileges string format
# mydb.*:INSERT,UPDATE/anotherdb.*:SELECT/yetanotherdb.*:ALL
- name: Example using login_unix_socket to connect to server
  mysql_user:
    name: root
    password: abc123
    login_unix_socket: /var/run/mysqld/mysqld.sock
- name: Example of skipping binary logging while adding user 'bob'
  mysql_user:
    name: bob
    password: 12345
    priv: "*.*:USAGE"
    state: present
    sql_log_bin: no
- name: Create user 'bob' authenticated with plugin 'AWSAuthenticationPlugin'
  mysql_user:
    name: bob
    plugin: AWSAuthenticationPlugin
    plugin_hash_string: RDS
    priv: '*.*:ALL'
    state: present
# Example .my.cnf file for setting the root password
# [client]