Werk #15881: saml2: addition of option to map attribute value to specific contact groups

Prior to this werk, the contactgroup attribute value had to match an existing Checkmk contact group name in order for the user to be assigned to the respective group. This werk introduces the multiple options for the attribute value to be mapped to multiple contact groups in case of a name match. The user can choose from following options:

* `No mapping`: The attribute value is not mapped to any contact group. * `Direct mapping`: Checkmk attempts to map the attribute value directly to a contact group with the same name. * `DN mapping`: Checkmk attemps to extract the CN name in the attribute value string and map it to a contact group same name * `Specific mapping`: The user can specify a mapping of attribute value to contact group name.

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