[SAML2] Only handle groups with External ID set

[Ionys320]

Describe the feature you'd like

Hi,
I would like to edit the {SAML2,OIDC}_USER_TO_GROUPS by adding a third value:

Value	Meaning
False	No sync between SAML2 groups and BookStack
True	Sync groups between SAML2 and BookStack using group name or external id
external-id	Sync groups between SAML2 and BookStack using the external id only

Describe the benefits this would bring to existing BookStack users

This allows the usage of custom groups on BookStack. For example, our instance is using the Admin group manually, so without any bind with SAML2 groups. But because of that, if we enable sync and especially SAML2_REMOVE_FROM_GROUPS, we are removed from the admins.

Can the goal of this request already be achieved via other means?

Create the group on the IdP side (SAML2 provider) and assign it to users. This disallow the management of the group on BookStack side.

Have you searched for an existing open/closed issue?

• I have searched for existing issues and none cover my fundamental request

How long have you been using BookStack?

1 to 5 years

Additional context

No response

Have you used generative AI/LLMs to create any thoughts in this request?

• This request only contains the thoughts & ideas of a human