Add option to cbbackupmgr to perform preflight check for backup

When using the Enterprise Backup tool, cbbackupmgr, in an automated production environment, we may want to gather information about the backup we are about to perform. This allows us to prompt for user intervention, perform other pre-backup actions or follow alternate logic in a calling/wrapper script, with the aim to avoid failed backups rather than deal with the aftermath.

The focus of the command should be on ease of use within an automated environment, though its outputs should be human readable, perhaps selectable with flags?

I envisaged this to begin a normal backup procedure but exit gracefully after valid data starts to be received in order to check for DCP rollbacks.

Whilst we cannot easily calculate the space required, we could find the number of changes we are expecting to stream, this can be used with some contextual information.

Some initial thoughts:

• Exit codes: Perhaps 0 for a incremental backup, 1 for full backup, other high or negative codes for hard errors
• JSON output for easy parsing
• Type of backup that would be ran (full/incremental)
• Number of expected changes in the backup
• Details of last backup in repo
• Reiterate the chosen options from the backup archive/repo, inc/exc buckets, etc