Record the date of last use of an OIDC client

[fishilico]

In the future, when there will be several OIDC clients, it will be useful when one has last been used, in order to know whether a key is still used for example. Currently django-oidc-provider records the creation date of a client, but not its date of last use (and the authgroupex endpoint records the date of last use).

This might requires some change in django-oidc-provider library.