This change streamlines the CryptoKey handling by utilizing the
codersdk package, thereby reducing code duplication and potentially
simplifying maintenance efforts in the future.

- Implement `cryptokeys.Keycache` interface in `CryptoKeyCache`.
- Introduce context management for graceful shutdowns.
- Simplify function signatures and improve concurrency handling.
- Ensure functions return errors when cache is closed.

- Use `cryptokeys.ErrKeyNotFound` for missing keys
- Use `cryptokeys.ErrKeyInvalid` for invalid keys
- Replace `xerrors` for streamlined error codes

- Replaced sync/atomic with sync.Mutex and sync.Cond for better
  concurrency control, avoiding frequent lock contention and reducing
  potential for data races.
- Removed separate fetch logic and integrated it into the main crypto
  key retrieval flow to handle concurrent fetches properly.
- Optimized the Close method to minimize wait time by broadcasting on
  cond to notify waiting fetches when closed.

The unused `cryptokeys.Keycache` interface implementation check was removed from the `CryptoKeyCache` struct, as it was unnecessary and did not contribute to functionality. This streamlines the code and adheres to best practices for interface checks.

The `latest` field in `CryptoKeyCache` was redundant and has been removed to simplify the code. Instead, the latest crypto key is now directly accessed from the `keys` map using `latestSequence`. This change reduces unnecessary state management and potential for data inconsistency.