fix: fix not working insecure login

[craicoverflow]

Use oauth2.HTTPClient to perform login
As the go-oidc provider can obtain the custom transport layer
from the context

Closes #

Verification Steps

Run login against a custom auth URL:

./rhoas login --auth-url "https://keycloakexternal.apps.akoserwa.ipt.integrations.rhmw.io/auth/realms/redhat-external" --insecure

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation change
• Other (please specify)

Checklist

• Documentation added for the feature
• CI and all relevant tests are passing
• Code Review completed
• Verified independently by reviewer

[craicoverflow]

cc @akoserwal

[akoserwal]

Verified. it works