Add a partition backup index

In Create a BackupPartition RPC (#6269 - closed) we are backing up the content of the partition as at the read snapshot LSN but depending on the type of restore, we will need to be able to find the correct backup to restore. This includes:

• Restore to a point-in-time
• Restore to a backup ID (for backup.rake)
• Restore a specific repository at a point-in-time

Each of these have different metadata that we will need to search in order to determine the correct backup to restore. The time, the backup ID and the relative path respectively. In addition we need to know the snapshot LSN so that after the full partition backup is applied we can replay archived logs.

We need to come up with a way that we can do this search in an efficient manner. Backups are stored using go cloud blob which only provide a basic prefix match search in order to find backups. In addition, if we want to support WORM in the future, then whatever solution we come up with here must be write once, without overwriting or updating files.