Add support for file group backup

[olahallengren]

I'm working on adding support for file group backup.

Usage:

EXECUTE dbo.DatabaseBackup
@Databases = 'WideWorldImporters',
@Directory = 'C:\Backup',
@BackupType = 'FULL',
@Verify = 'Y',
@FileGroups = 'WideWorldImporters.USERDATA, WideWorldImporters.USERDATA2',
@NumberOfFiles = 4

Outstanding issues:

• The primary filegroup cannot be backed up if the database is in the SIMPLE recovery model.
• The primary filegroup and any memory-optimized filegroup must be backed up together.

I need to add handling of these cases.