[SAML Groups] Allow linking accounts/domains to SAML groups

Most SAML IdPs are able to provide group information for the authenticated user in the form of a SAML attribute (with many values for each group the user is in). It would be useful to be able to use the groups in the same way that LDAP groups can, such as to give a user access to certain accounts.

Here is an example SAML response from AWS Identity Center showing how groups are formatted.

```xml


ourdomainhere.internal//S-1-5-21-3122984950-2570546592-4150994639-1732
ourdomainhere.internal//S-1-5-21-3122984950-2570546592-4150994639-2026
ourdomainhere.internal//S-1-5-21-3122984950-2570546592-4150994639-1814
ourdomainhere.internal//S-1-5-21-3122984950-2570546592-4150994639-1722
ourdomainhere.internal//S-1-5-21-3122984950-2570546592-4150994639-1748
ourdomainhere.internal//S-1-5-21-3122984950-2570546592-4150994639-1730
ourdomainhere.internal//S-1-5-21-3122984950-2570546592-4150994639-1953
ourdomainhere.internal//S-1-5-21-3122984950-2570546592-4150994639-1836


rhys@ourdomainhere.internal


```

This would be a useful feature to have as with more companies moving to a more "serverless" / "cloud-native" (insert your favourite buzzword here) it is becoming less common to an easy way to connect to the user directory with LDAP.

Start by locating the existing LDAP group handling and the SAML attribute parsing or authentication entry points. Define how the multi-valued SAML groups attribute should map to account or domain access, then verify that those links provide the same access behavior as LDAP groups.

Written by the indexing model from the issue text.