Add state migrator for external_member_group_ids in Identity Group

[vinay-gopalan]

Description

Adds a state migrator for the external_member_group_ids parameter that was added to the Identity Group resource in v3.10.0. Since the parameter is an internal constant with a default value, it should be set to users' TF configs when they upgrade to the new version. Since it was previously not being set for users upon an upgrade, the regression led to the value being null in the TF state. This PR ensures that upon upgrading, if the TF state value is null, set it to the desired default of false

Closes #1690

Checklist

• Added CHANGELOG entry (only for user-facing changes)
• Acceptance tests where run against all supported Vault Versions

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestSecretsAuthDisableRemountUpgradeV0'
=== RUN   TestSecretsAuthDisableRemountUpgradeV0
=== RUN   TestSecretsAuthDisableRemountUpgradeV0/basic
--- PASS: TestSecretsAuthDisableRemountUpgradeV0 (0.00s)
    --- PASS: TestSecretsAuthDisableRemountUpgradeV0/basic (0.00s)
PASS

[fairclothjm]

Thanks!