New-AzVMSqlServerAutoBackupConfig - Specify days for full backups

## Description of the new feature
Currently, when creating an autobackup config and specifying FullBackupFrequency = Weekly, backup schedule will be set to execute on all 7 days of the week, making it functionally the same as FullBackupFrequency = Daily. There is no (clear) way to set specific days to run full backup.
Compared to the T-SQL stored procedure managed_backup.sp_backup_config_schedule which supports the parameter @days_of_week, where you can specify the days like 'Sunday|Tuesday|Thursday'

## Proposed implementation details (optional)
Add parameter to the cmdlet to specify specific days of the week to execute full backup, eg -FullBackupWeekDays Sunday,Tuesday,Thursday