Allow OIDC connector to remap claims even if the original claims are not available

[nabokihms]

Is your feature request related to a problem?

There is a problem, that users cannot map custom OIDC token claim to the dex claim.
For example, the original token has two claims:

{
 "email": "[email protected]",
 "preferred_email": "[email protected]",
  ...
}

Apparently, a user wants to use preferred_email as an email claim in Dex but has no opportunity to do it, because the email claim is present in the token.

See corresponding code lines:
https://github.com/dexidp/dex/blob/master/connector/oidc/oidc.go#L306-L310

Describe the solution you'd like to see

Allow users to remap claims even if the standard one exists in the token. There are many OIDC providers that allow claims customization. I see no objections to allowing users to manage claims as they want.

Sadly, making this strategy enabled by default is a breaking change.

Describe alternatives you've considered

Middlewares? :)

Additional context

Was discussed here #2158 (comment)