Allow Data Masking to be applied/exempted by DB Role

Allow the "applies_to" and "exempted" options in the Data Masking rules to allow DB Roles in addition to specific users. Alternately create new options that allow the use of DB Roles.

When managing 100's or 1,000's of users it is impractical to have to go in and change the Data Masking rules when adding/altering relationships to these rules. Most data access is handled through DB Roles when dealing with large numbers of users and the Data Masking should follow this pattern.

We heavily leverage the Data Masking feature in a competing product and it will be a hard sell to endorse moving to MariaDB without a comparable feature.