Add Folder Credentials Provider

[timja]

TODO:

• Update cache to handle credentials with a context (folder level credentials)
• Documentation
• Possibly move files in a separate PR
• Refactor code to share fetching of Key Vault creds
• Prevent Add credentials showing up on folder credentials provider (not sure why it's showing atm)
• Look at how this works with nested folders
• Possibly add an ID that can be prefixed to uniquely ID a cred, could also be a vault name
• Tests