feat(policy): Add UnsafeUpdateKey rpc. (#3728)

## Summary

- Add a new `UnsafeUpdateKey` proto, to enable the following flows:
   - Switching a key from `public_key` <-> `remote`
   - Switching provider configurations for a key

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## Summary by CodeRabbit

- **New Features**
- Added the `UnsafeUpdateKey` API for updating an existing key using its
ID, target key mode, and optional provider configuration.
  - The API response returns the updated key details.
  - Added client and server interface support for the new operation.

- **Documentation**
- Updated gRPC and OpenAPI documentation with request fields, response
schemas, endpoint details, and safety warnings.

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Signed-off-by: Chris Reed <creed@virtru.com>

Author: c-r33d