fix: Use URN returned by the API for me_identity_group

[amstuta]

Description

• Add URN in datasource ovh_me_identity_group
• Use URN returned by the API in resource ovh_me_identity_group

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [x] Improvement (improve existing resource(s) or datasource(s))
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation update

How Has This Been Tested?

• [x] Test A: make testacc TESTARGS="-run TestAccMeIdentityGroupDataSource_basic"
• [x] Test B: make testacc TESTARGS="-run TestAccMeIdentityGroup_basic"