- Simplify naming by renaming `DBKeyCache` to `DBCache`.
- Introduce `DBCacheOption` for flexible clock configuration.
- Ensure consistent clock handling across the methods.

Refactored the CryptoKey handling logic to leverage the codersdk
package. This enhances consistency and modularity by centralizing
CryptoKey management and related operations. The refactoring also
includes renaming functions to better reflect their functionality
and using the db2sdk package for database to SDK conversions.

- Simplifies conversion and improves consistency by using
  a common package for translating database keys to SDK keys.
- Improves test reliability and code maintainability by centralizing
  the conversion process.

Introduce a timer-based cache invalidation system in DBCache to enhance reliability. The new implementation minimizes cache invalidation race conditions during key fetching, ensuring consistent cache state management. Additions include a 'Close' method for releasing resources, such as timers, and test improvements for timer behavior validation.

Introduce `closed` state to the DBCache to ensure operations such
as Verifying and Signing are safely terminated when the cache is
closed. This update prevents resource access and ensures proper
shutdown sequences, enhancing system reliability.