[FIX] resource/vault_identity_group external member_entity_ids & member_group_ids

[gavinelder]

Summary

Follow up from Linked PRS #1061 & #1054 to ensure member_entity_ids & member_group_ids are only managed on internal groups

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Closes #1133

Release note for CHANGELOG:

resource/vault_identity_group: Fix bug where member_entity_ids &  member_group_ids  were attempted to be managed on external identity groups.

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestAccXXX'

...

[hashicorp-cla]

All committers have signed the CLA.