Support cache invalidation

[HanadaLee]

Currently, this module's purge method deletes cached files.
However, in CDNs or other specific scenarios, it's often only necessary to invalidate the cache (set it to expire) rather than directly deleting it to reduce unnecessary upstream transmissions. Invalidated caches can be re-validated upstream (304).

Function Description:

Introducing the directive:

*_cache_purge_type delete | invalidate;

It is recommended that the value of the directive include variables, so that control can be passed in via request headers, etc.

*_cache_purge_type $http_purge_type;