Ability to delete/retrieve all items in a PrivateTempStore for a given owner or all owners

Problem/Motivation

The

PrivateTempStore

works nice and well for creating a store, adding items to that store and then deleting individual items from the store. However, it does not provide an API to retrieve all the items from the store or deleting them. Although they are temporary, there is still a valid use case in my opinion for the possibility to delete all the items stored in a given collection for the given user. For example, a multistep form which has fields prepopulated with values in the store. Submitting the last part of the form should clear the store and save the data somewhere else.

Proposed resolution

Add 2 new public methods to delete and retrieve all the items in the store.

Remaining tasks

Write patch and test.

User interface changes

None.

API changes

Ability to delete all items in the store or retrieving all of them.