add ability to read an organization membership by org membership id

This will be useful in case you need to read all organization
memberships and pull out information such as emails of all users so that
it can be used to create downstream resources, example:

```terraform
data "tfe_organization_members" "users" {
  organization = var.tfc_org
}

data "tfe_organization_membership" "test" {
  for_each = toset([
    for index, member in data.tfe_organization_members.users.members :
    member.organization_membership_id
  ])
  organization               = var.tfc_org
  organization_membership_id = each.value
}

output "user_emails" {
  value =  [
    for index, member in data.tfe_organization_membership.test :
    member.email
  ]
}
```

Related issue: #871

Author: AmineChikhaoui

internal/provider/data_source_organization_membership.go

@@ -67,6 +67,14 @@ func dataSourceTFEOrganizationMembershipRead(d *schema.ResourceData, meta interf
 
 		d.SetId(orgMember.ID)
 	} else {
+		orgMember, err := fetchOrganizationMemberById(context.Background(), config.Client, orgMemberID)
+		if err != nil {
+			return fmt.Errorf(
+				"could not find organization membership (%s) for organization %s: %w", orgMemberID, organization, err,
+			)
+		}
+
+		d.Set("email", orgMember.Email)
 		d.SetId(orgMemberID)
 	}
 

internal/provider/organization_members_helpers.go

@@ -103,3 +103,15 @@ func fetchOrganizationMemberByNameOrEmail(ctx context.Context, client *tfe.Clien
 
 	return nil, tfe.ErrResourceNotFound
 }
+
+func fetchOrganizationMemberById(ctx context.Context, client *tfe.Client, orgMemberId string) (*tfe.OrganizationMembership, error) {
+	orgMember, err := client.OrganizationMemberships.Read(ctx, orgMemberId)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read organization membership %s: %w", orgMemberId, err)
+	}
+	if orgMember != nil {
+		return orgMember, nil
+	}
+
+	return nil, tfe.ErrResourceNotFound
+}