feat: oidc accept groups as string

[susguzman]

Overview

The current OIDC connector only support the groups claim as an array, but if the response is a string, because the user only has one group, the groups cannot be mapped. I was configuring Dex with an OIDC provider that response a string when the user has only one group, so it's needed a way to handle this scenarios.

What this PR does / why we need it

• Allows the group claim be a string and an array.
• It's need because there are some providers that response with a string when the user only have one group.

Does this PR introduce a user-facing change?

NONE

OIDC Connector: Support for groups as a string.

[nabokihms]

It seems ok to me to support converting a string to a group/groups.
@susguzman, could you please give us an example of the provider that responds with a string if a user has only one group?

[nabokihms]

My bad, accidently linked unrelated PR.

[nabokihms]

Closed by #2639