Restrict which Google accounts are allowed to be set as browser primary accounts in Brave

Contains a regular expression which is used to determine which Google accounts can be set as browser primary accounts in Brave (i.e. the account that is chosen during the Sync opt-in flow). An appropriate error is displayed if a user tries to set a browser primary account with a username that does not match this pattern. If this policy is left not set or blank, then the user can set any Google account as a browser primary account in Brave. Example value: .*@example\.com