Fix some issues with vault_identity_group

[lawliet89]

• policies were not updated properly from reading the resource from Vault
• Update fields policies, member_group_ids, and member_entity_ids to be of schema.TypeSet instead. These lists should contain unique items and the order should not matter when calculating the diff
• Handle member_entity_ids for external groups (otherwise we get errors like error updating IdentityGroup "a6c92cfa-d0b0-8d61-7122-a59cf1123a45": cannot set 'member_entity_ids' on external groups)

Other fixes:

• Fix typo in vault_identity_group_alias

Can't really write tests for the diff suppression without "logging in" with a real auth backend. It works in my local tests, though.

Before:

  ~ vault_identity_group.admins
      member_entity_ids.#: "1" => "0"
      member_entity_ids.0: "2b83bd73-954c-7b4d-b102-bec997008baf" => ""

  ~ vault_identity_group.users
      member_entity_ids.#: "1" => "0"
      member_entity_ids.0: "2b83bd73-954c-7b4d-b102-bec997008baf" => ""

After:

No changes. Infrastructure is up-to-date.

[tyrannosaurus-becks]

@lawliet89 thank you for working on this! Looks great!

One question - should there be any doc updates accompanying this?

[lawliet89]

The entire resource is undocumented. Is it ok for that to be done in a separate PR?

[lawliet89]

That's done in #311 .

[tyrannosaurus-becks]

@lawliet89 I saw and approved the docs PR. Thank you!