Fix username extraction on OIDC tokens with a group scope

[yahavi]

• [x] All tests passed. If this feature is not already covered by the tests, I added new tests.
• [x] All static analysis checks passed.
• [x] This pull request is on the dev branch.

• [x] I used gofmt for formatting the code before submitting the pull request.

  Fix https://github.com/jfrog/setup-jfrog-cli/issues/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.