Add RefreshOIDCToken middleware

[GermanoGuerrini]

I noticed that the library was missing the ability to use any refresh token issued by the provider.
It can be used in place of a silent user re-authentication (which does not work for the provider I was working with).
Information can be found here.
Now, while the implementation in this pull request works, it misses a proper provider response validation as described in the OpenID Specs.
That would require a serious refactoring in order to store extra information from the original response in a safer place than the user session, like the database, which currently isn't involved at all.
Plus, the validation should be factored out from the OIDCAuthenticationBackend to be re-used wherever needed.
Is there any interest in such contribution?

[codecov-io]

Codecov Report

Merging #301 into master will increase coverage by 0.66%.
The diff coverage is 97.22%.

@@            Coverage Diff             @@
##           master     #301      +/-   ##
==========================================
+ Coverage    88.1%   88.76%   +0.66%     
==========================================
  Files           7        7              
  Lines         437      463      +26     
==========================================
+ Hits          385      411      +26     
  Misses         52       52

Impacted Files	Coverage Δ
mozilla_django_oidc/auth.py	94.05% <100%> (+0.06%)	⬆️
mozilla_django_oidc/middleware.py	92.68% <96.29%> (+3.02%)	⬆️

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[titilambert]

Any news on that ?

[Galtozzy]

Any updates?