fix(IdProviderFactory): make client_id and client_secret optional with .get()

[aldbr]

Vladimir and I sometimes play with the Computing Elements and need to get tokens from the TokenManager. It was working until that commit: bedcfb0

Now we get the following error:

>           pDict["client_id"] = pDict[f"{client_name_prefix}client_id"]
                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
E           KeyError: 'client_id'

Why? Because the TokenManagerClient is obviously not expected to store the client_id and client_secret but might need all the public info to cache the tokens (name, scope, issuer, ...). Therefore, it's calling IdProviderFactory.getIdProvider(), which know try to extract the secret fields.

Here I added a simple unit test to avoid regression and make these client_id and client_secret optional.

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*Resources
FIX: make client_id and secret optional to avoid KeyError exceptions.
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