adapters and feldera-types: secrets rework

[snkas]

• Secret references can now be used in the value strings of the connector configuration JSON for transport.config and format.config
• New Kubernetes Secret reference syntax: ${secret:kubernetes:<name>/<data key>}
• Syntax allows selecting a specific data key of a Kubernetes Secret
• Potential expansion to more secret providers in the future
• Documentation page for secret references
• Secrets are now mounted under /etc/feldera-secrets

Remaining tasks:

• [ ] Pass over connector printing in adapters
• [x] Check that documentation works
• [x] Determine interaction with suspend-and-resume

[snkas]

Updated PR to incorporate feedback from @blp :

• Resolution limited to transport.config and format.config
• Use try_exists for file check
• Add error kind to the resolution error
• Typo fix

[brurucy]

Really looking forward to this