fix(auth): render username in top bar

[andrewazores]

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Fixes https://github.com/cryostatio/cryostat3/issues/489 Related to https://github.com/cryostatio/cryostat3/pull/490

Description of the change:

If the response from the server includes the Gap-Auth header, use that to determine the user's logged in username. If not, fall back to using the POST /api/v2.1/auth response body. If anything is blank or fails then use the empty string, which the UI already renders as the anonymous user icon.

How to manually test:

1. Check out cryostat3 and build with this PR
2. ./smoketest.bash -O
3. Verify that the username user appears on the top bar
4. Restart smoketest as ./smoketest.bash -Op to disable the auth proxy
5. Verify that the anonymous user icon appears on the top bar

[aali309]

/build_test

[github-actions[bot]]

Test image available:

CRYOSTAT_IMAGE=ghcr.io/cryostatio/cryostat-web:pr-1264-c77ea3822e750350a4c14cc14ff9dc0d4d49640a bash smoketest.bash # then open http://localhost:8080