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/examples/data-sources/sentry_notification_target/data-source.tf

@@ -0,0 +1,16 @@
+# secret comes back unredacted (see the sentry_notification_target
+# resource's schema doc comment) -- this data source's "secret"
+# attribute is Sensitive for the same reason.
+data "sentry_notification_target" "ops_webhook" {
+  id = "an-existing-target-id"
+}
+
+resource "sentry_alert_rule" "checkout_5xx" {
+  name                    = "Checkout 5xx spike"
+  query                   = "service=checkout status>=500 | stats count"
+  condition_type          = "threshold"
+  comparator              = "gt"
+  threshold_value         = 50
+  eval_interval_seconds   = 60
+  notification_target_id  = data.sentry_notification_target.ops_webhook.id
+}