Add sentry_alert_rule, the Terraform provider's second resource

Confirmed with the project owner first: alerting's REST API has no
PUT /rules/{id} at all -- confirmed down to rulestore.Store, which has
Create/List/Get/Delete but no Update method to even wire one to, a real
pre-existing gap in alerting's own API, not something new to this task.
Decided to model sentry_alert_rule as create/destroy only rather than
fake an in-place update via delete-then-recreate inside the resource:
every attribute carries a RequiresReplace plan modifier, so a config
change destroys and recreates the rule, surfacing in the plan output the
real side effect that has (alert_state/delivery-log continuity resets)
instead of hiding it. Adding a real PUT /rules/{id} to alerting would
remove this constraint but is a change to a different module's REST
API, out of scope here.

internal/provider/client.go's new rule type and createRule/getRule/
deleteRule methods talk the exact same JSON contract
sentryctl alerts apply already uses against alerting/internal/httpapi.
GET /rules/{id} actually returns rulestore.RuleWithState (Rule's fields
promoted via anonymous embedding, plus a "state" object) -- the local
rule type has no field for "state" by design, and a new client test
proves that extra key doesn't break parsing.

alerting is a genuinely separate service from api (its own base URL),
so this needed the provider to talk to more than one Sentry service for
the first time: providerData now wraps two *client instances (api,
alerting), with a new alerting_endpoint provider attribute defaulting
the same way sentryctl's --alerting-api/$SENTRYCTL_ALERTING_API_URL
does. dashboardResource's Configure updated to pull .api out of the new
wrapper type instead of a bare *client.

Schema mirrors sentry_dashboard's established pattern: comparator/
threshold_value/renotify_interval_minutes stay nullable (only meaningful
for threshold-condition rules), enabled/for_minutes/query_language are
Optional+Computed with a Terraform-side default matching the API's own
default (true/0/"") rather than leaving the API as sole source of truth
the way dashboard's default_earliest/default_latest deliberately do --
these three have no *pointer* type in the API's Rule struct, so their
"default when omitted" is unconditional, not a real API-side default
that could drift independently.

Verified: client tests are real httptest.Server round trips (same
pattern as sentry_dashboard's). Schema validation needs no Terraform
binary. TestAccAlertRuleResource_basic is a real acceptance test,
skip-gated by TF_ACC same as the dashboard one, including a
plancheck.ExpectResourceAction assertion that a config change actually
plans destroy-then-create -- the concrete, checked version of the
"create/destroy only" design decision, not just a comment. Not run
against a live stack in this environment, same disclosed gap as
everything else Docker-gated in this repo.

terraform/examples/resources/sentry_alert_rule/import.sh

@@ -0,0 +1 @@
+terraform import sentry_alert_rule.checkout_5xx <rule-id>

terraform/examples/resources/sentry_alert_rule/resource.tf

@@ -0,0 +1,19 @@
+# notification_target_id has to name an already-existing target --
+# no sentry_notification_target resource exists yet (see the provider
+# README), so create one via sentryctl/curl/the web UI first and pass
+# its id in here.
+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
+  for_minutes            = 5
+  notification_target_id = "target-abc123"
+}
+
+# Create/destroy only -- alerting has no PUT /rules/{id} today, so
+# changing any attribute above (including this resource block itself)
+# destroys and recreates the rule rather than updating it in place. See
+# the provider README for why.