Support providing credentials via secret in hook

[hossainemruz]

Stash backup and restore hooks opens a huge possible use cases.
For example,

• users can stop a particular application before backup and then resume it after backup is completed
• user can cleanup corrupted data before restore.

Another nice example is,

• a user can send notification to a webhook before and after backup/restore process.

However, the last use-case may require providing authorization credentials as well as TLS certificates. Currently, it is not possible to provide them safely.

[tamalsaha]

kmodules/prober#8