Fix data assignment APIs

[Rajesh-Pirati]

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Relates OR Closes #2348

Output from acceptance testing:

=== RUN   TestAccIBMIAMPolicyAssignmentsDataSourceBasic
--- PASS: TestAccIBMIAMPolicyAssignmentsDataSourceBasic (16.26s)
PASS
ok  	github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/iampolicy	18.218s

...

[Rajesh-Pirati]

terraform to display the assignment details

terraform show
# data.ibm_iam_policy_assignment.policy_assignment:
data "ibm_iam_policy_assignment" "policy_assignment" {
    account_id          = "xxxxx"
    assignment_id       = "policyAssignment-1181cc96-bd6b-44f6-a56b-c2a3907b2b8d"
    created_at          = "2023-07-19T23:42:37.472Z"
    created_by_id       = "IBMid-xxxxx"
    href                = "https://iam.test.cloud.ibm.com/v1/policy_assignments/policyAssignment-1181cc96-bd6b-44f6-a56b-c2a3907b2b8d"
    id                  = "policyAssignment-1181cc96-bd6b-44f6-a56b-c2a3907b2b8d"
    last_modified_at    = "2024-04-10T20:16:07.973Z"
    last_modified_by_id = "migration"
    options             = [
        {
            root = [
                {
                    assignment_id = "TemplateAssignment-fb668001-d73b-48d6-8f88-c0d326e1573c"
                    requester_id  = "IBMid-xxxx"
                    template      = []
                },
            ]
        },
    ]
    resources           = [
        {
            policy = [
                {
                    error_message    = []
                    resource_created = [
                        {
                            id = "5e5bf02b-45a8-47f8-a4ab-d4789f2beddc"
                        },
                    ]
                    status           = ""
                },
            ]
            target = {
                "id" = "xxxxx"
            }
        },
    ]
    subject             = {
        "id"   = "iam-Profile-9195d7e5-bad6-4365-9226-5093c61a8e9e"
        "type" = "iam_id"
    }
    target              = {
        "id"   = "xxxxx"
        "type" = "Account"
    }
    template            = {
        "id"      = "policyTemplate-e95df540-404b-4630-84ca-82ea4b5f7959"
        "version" = "1"
    }
    version             = "1.0"
}