feat: implement dynamic cache tags

[VariableVic]

Implements dynamic cache tags throughout the storefront. This ensures that the cache will be invalidated for the current client only when server data mutates.

• Added some logic to Nextjs middleware that generates a unique id and sets it in a _medusa_cache_id cookie.
• Added a getCacheTag() helper function which takes in a string input. This will return a dynamic cache tag string in the format {input}-{_medusa_cache_id}. This is useful when revalidating tags after performing a mutation.
• Added a getCacheHeaders() helper function which takes in a string input. This will return the next: {} headers to assign a dynamic cache tag to a GET request.