SqlServerRole: Add support for nested roles

[nabrond]

Details of the scenario you tried and the problem that is occurring

SQL Server supports both logins and user-defined server roles being members of a server role. The SqlServerRole resource should be enhanced to add this support. Currently, the resource checks whether all Members, MembersToInclude, or MembersToExclude are valid Logins on the system.

Verbose logs showing the problem

N/A

Suggested solution to the issue

Instead of solely checking for a valid Login, the resource should also validate whether it matches a Role defined in the instance.

The DSC configuration that is used to reproduce the issue (as detailed as possible)

SqlServerRole 'Role-DBAdmins'
{
    Ensure = 'Present'
    ServerRoleName = 'DBAdmins'
    ServerName = 'sql.contoso.com'
    PsDscRunAsCredential = $SqlAdministratorCredential
}

SqlServerRole 'Role-sysadmin'
{
    Ensure = 'Present'
    ServerRoleName = 'sysadmin'
    ServerName = 'sql.constoso.com'
    MembersToInclude = 'DBAdmins'
    PsDscRunAsCredential = $SqlAdministratorCredential
}

SQL Server edition and version the target node is running

SQL 2012+

SQL Server PowerShell modules present on the target node

SQLPS
SqlServer
SqlServerDsc

The operating system the target node is running

N/A

Version and build of PowerShell the target node is running

N/A

Version of the DSC module that was used ('dev' if using current dev branch)

dev

[johlju]

Sounds like a good addition.