Onchain logs for KMS replication

## Summary
This is a [discussion](https://github.com/Dstack-TEE/dstack/pull/91#discussion_r1937226966) about whether KMS instances should be whitelisted on-chain before being allowed to onboard, for improved transparency and security.

## Current Approach
- Device-ID (implemented as a hash of the PPID) is whitelisted

## Proposed Enhancement
- Add on-chain whitelisting for each KMS instance before onboarding is permitted
- Create an on-chain trace/log of onboarding events when instances request KMS keys

## Rationale
- Onboarding to KMS (sharing the secret) is considered a highly security-critical component
- On-chain logging would increase transparency and traceability
- Could potentially implement an interactive on-chain challenge to ensure up-to-date view of the chain

## Benefits
- Improved visibility into KMS operations
- Public verification of legitimate KMS instances
- Transparent audit trail of key sharing events

## Considerations
- Would require additional on-chain operations
- Need to balance transparency with performance and gas costs