Add OIDC claim names options

[fen4o]

Some identity providers (auth0 for example) do not allow to set the groups claims (https://auth0.com/docs/secure/tokens/json-web-tokens/create-custom-claims) and administrators must use custom claims names and add them in the id token.

This commit adds the following configuration options:

• oidc.groups_claim to set the groups claim name
• oidc.email_claim to set the email claim name

All claims default to the previous values for backwards compatibility.

The groups claim can now also accept []string or string as some providers might return only a string response instead of array.

• read the CONTRIBUTING guidelines
• raised a GitHub issue or discussed it on the projects chat beforehand
• added unit tests
• added integration tests
• updated documentation if needed
• updated CHANGELOG.md

Fixes #1114

[FStelzer]

thanks for this. i'll be able to test it in a few days.
This is also needed for O365/AD OIDC Auth when you should use userPrincipalName instead of email. The latter might be editable by the user themself.

[IamTaoChen]

#1934 I add a clamis_map for this