Adds support for cache settings configuration.

Adds support for configuring (through the admin settings page) new cache status and TTL settings.

Updates OptionsManager to provide easy access and default values for the new settings.
Adds constant values for the cache status setting flags.
Adds cache status option support.

Content retrieval now honors the cache status option configuration.
Implements a configurable content cache entry expiry.

This implementation uses the value specified in the cache TTL plugin option.
Updates cache key generation to properly prefix with 'getgit' plugin name.
Updates setting page input parsing to convert TTL value to an int before storing it as an option value.
Improves input validation for the cache TTL setting.

Input validation for the cache TTL setting now include checks to ensure that the provided value is greater than zero.