Add read-only data sources for all three Terraform resources

Mechanical, low-risk follow-up -- no new architectural question, no new
external service, no new write path. Each of sentry_dashboard,
sentry_alert_rule, and sentry_notification_target gets a matching data
source: a single Required id attribute in, every other attribute
Computed out, backed by the exact same getDashboard/getRule/
getNotificationTarget client methods and dashboardModelFromAPI/
alertRuleModelFromAPI/notificationTargetModelFromAPI conversion
functions the resources already use and already have tests for -- these
data sources add no new client code at all, just a thin
datasource.DataSource wrapper reusing what Create/Read/Update/Delete
already exercise.

sentry_notification_target's data source carries the same secret
caveat its resource does (Sensitive, but alerting's GET /targets/{id}
returns it unredacted, so it's a real plaintext value in Terraform
state) -- named again here rather than assumed obvious from the
resource's own docs.

Verified: provider_test.go's new schema-validation tests confirm each
data source's id is Required and everything else Computed, no
Terraform binary needed. Three new real acceptance tests
(TestAccDashboardDataSource_basic and its two siblings) each create a
resource then look it up via the matching data source, using
resource.TestCheckResourceAttrPair to prove the data source's Read
actually agrees with what the resource wrote -- not just that both
compile. Skip-gated by TF_ACC same as the existing six acceptance
tests, and needs the same live api/alerting services this environment
has no Docker access to bring up, so not run here -- same disclosed gap
as everything else Docker-gated in this repo.

terraform/internal/provider/dashboard_data_source_test.go

@@ -0,0 +1,38 @@
+package provider
+
+import (
+	"testing"
+
+	"github.com/hashicorp/terraform-plugin-testing/helper/resource"
+)
+
+// Same skip-gated-not-faked posture as TestAccDashboardResource_basic --
+// see that test's doc comment.
+func TestAccDashboardDataSource_basic(t *testing.T) {
+	resource.Test(t, resource.TestCase{
+		ProtoV6ProviderFactories: testAccProtoV6ProviderFactories,
+		Steps: []resource.TestStep{
+			{
+				Config: `
+provider "sentry" {
+  endpoint = "http://localhost:8080"
+}
+
+resource "sentry_dashboard" "test" {
+  name        = "Data Source Test Dashboard"
+  description = "created by TestAccDashboardDataSource_basic"
+}
+
+data "sentry_dashboard" "test" {
+  id = sentry_dashboard.test.id
+}
+`,
+				Check: resource.ComposeAggregateTestCheckFunc(
+					resource.TestCheckResourceAttrPair("data.sentry_dashboard.test", "name", "sentry_dashboard.test", "name"),
+					resource.TestCheckResourceAttrPair("data.sentry_dashboard.test", "tenant_id", "sentry_dashboard.test", "tenant_id"),
+					resource.TestCheckResourceAttrPair("data.sentry_dashboard.test", "default_earliest", "sentry_dashboard.test", "default_earliest"),
+				),
+			},
+		},
+	})
+}