Add aws_identitystore_group_memberships data source

[liath]

Description

Adds a data source for listing the members of an Identity Store Group. Part of a few PRs to cover #26770.

Relations

Relates #26770

References

https://docs.aws.amazon.com/cli/latest/reference/identitystore/list-group-memberships.html
https://docs.aws.amazon.com/singlesignon/latest/IdentityStoreAPIReference/API_ListGroupMemberships.html

Output from Acceptance Testing

$ make testacc TESTS=TestAccIdentityStoreGroupMembershipsDataSource PKG=identitystore
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go1.22.2 test ./internal/service/identitystore/... -v -count 1 -parallel 20 -run='TestAccIdentityStoreGroupMembershipsDataSource'  -timeout 360m
=== RUN   TestAccIdentityStoreGroupMembershipsDataSource_basic
=== PAUSE TestAccIdentityStoreGroupMembershipsDataSource_basic
=== CONT  TestAccIdentityStoreGroupMembershipsDataSource_basic
--- PASS: TestAccIdentityStoreGroupMembershipsDataSource_basic (20.24s)
PASS
ok      github.com/hashicorp/terraform-provider-aws/internal/service/identitystore      31.620s

[github-actions]

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[liath]

I went with the arrays of each result attribute style from ssm_parameters_by_path vs one array for each result like they come from the API (ala iam_access_keys). Happy to switch if that wasn't the right call but I didn't see anything in the docs to prefer one over the other. I figured the majority of callers only care about the member_ids field anyways so we might as well make that as easy to consume as possible.

[liath]

Migrated to the plugin framework as @aristosvo has done for #31681 / #36993, hopefully that helps it get merged.