feat(queue/sql): add subscriber with partition leasing

## Why?
Implement the Subscriber interface with distributed partition leasing to enable reliable message consumption across multiple workers.

## What?
- Subscriber with automatic partition discovery and lease acquisition
- Per-partition polling with ack/nack support
- DLQ handling for messages exceeding retry limits
- Lease renewal and graceful shutdown
- Fast test execution (~4s) with comprehensive coverage

## Test Plan
- Message delivery and ordering verified
- Ack/nack operations work correctly
- DLQ handling tested (max retries exceeded)
- Multi-partition and multi-consumer scenarios validated
- Lease management (acquire, renew, release) tested

Author: behinddwalls

extensions/queue/sql/BUILD.bazel

@@ -9,12 +9,14 @@ go_library(
         "offset_store.go",
         "partition_lease_store.go",
         "publisher.go",
+        "subscriber.go",
         "test_helpers.go",
     ],
     importpath = "github.com/uber/submitqueue/extensions/queue/sql",
     visibility = ["//visibility:public"],
     deps = [
         "//entities/queue",
+        "//extensions/queue",
         "@com_github_dolthub_go_mysql_server//:go-mysql-server",
         "@com_github_dolthub_go_mysql_server//memory",
         "@com_github_dolthub_go_mysql_server//server",
@@ -36,11 +38,13 @@ go_test(
         "offset_store_test.go",
         "partition_lease_store_test.go",
         "publisher_test.go",
+        "subscriber_test.go",
     ],
     data = ["//schema/queue/mysql:schema.sql"],
     embed = [":sql"],
     deps = [
         "//entities/queue",
+        "//extensions/queue",
         "@com_github_go_sql_driver_mysql//:mysql",
         "@com_github_stretchr_testify//assert",
         "@com_github_stretchr_testify//require",