Fix username extraction on OIDC tokens with a group scope

[yahavi]

• All tests passed. If this feature is not already covered by the tests, I added new tests.
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Fix jfrog/setup-jfrog-cli#146.

Previously, we assumed all JWT tokens followed this subject structure:

jfac@<some-characters>/users/<user-name>

However, OIDC tokens with group scopes might follow this subject structure instead:

<provider-name>/<identity-mapping>

This structure can serve as a standard username.