Add GetAndTouch Method to Client

[ghrushneshr25]

This pull request introduces the GetAndTouch method to the Client struct. The GetAndTouch method retrieves a cache item by its key and updates its expiration time, ensuring that the item remains valid in the cache for the specified duration.

Changes:

Added Method: GetAndTouch
This method calls getAndTouchFromAddr to perform the cache retrieval and expiration time update.
If no item is found (i.e., item is nil), it sets the error to ErrCacheMiss.

Added Method: getAndTouchFromAddr
This method constructs and sends the gat command to the cache server, including the expiration time and key.
It handles the server response and updates the item through the provided callback function.

Usage:

Use GetAndTouch to fetch a cache item and update its expiration time.
Pass a negative expiration value to instantly delete the key from the cache after retrieving it. This approach saves two client calls—one for reading and one for deleting.

How to Use:
item, err := client.GetAndTouch("myKey", 3600)