Subscription types: Map subscriptions and Shared Poll subscriptions

[FZambia]

Summary

This PR extends Centrifugo with support for the two new subscription types introduced in the centrifuge library centrifugal/centrifuge#565 — Map subscriptions and Shared Poll subscriptions — and adds a PostgreSQL MapBroker as a first-class engine option.

Map subscriptions

Integrates centrifuge's map subscription model into Centrifugo's configuration, proxy, and API layers.

• Namespace-level subscription_type — each namespace declares its type: stream (default), map, map_clients, map_users, or shared_poll. Only matching subscription types are allowed per namespace.
• map config block on namespace — controls mode (ephemeral/durable/persistent), key_ttl, ordered, stream_size, stream_ttl, external_state, publish/remove permissions, and proxy settings.
• Map presence channels — map_clients_presence_channel_prefix and map_users_presence_channel_prefix on stream namespaces allow tracking connection/user presence via MapBroker in a derived channel.
• Server API — new endpoints: MapPublish, MapRemove, MapReadState, MapReadStream, MapStats, MapClear (HTTP + gRPC).
• Proxy protocol — new MapPublish, MapRemove proxy types (HTTP + gRPC) for delegating map writes to the application backend.
• Track signature verification — shared poll track requests are authorized via a signature string; Centrifugo verifies it server-side with key rotation support.

Shared Poll subscriptions

• shared_poll config block on namespace — proxy_name, refresh_interval, refresh_batch_size, max_keys_per_connection, mode (versionless/versioned), publish_enabled (fast-track), and tuning options.
• Proxy protocol — new SharedPollRefresh proxy type (HTTP + gRPC) that Centrifugo calls on each poll cycle with the aggregated key set. The backend returns current data per key.
• SharedPollPublish API — server API endpoint for fast-track publication to shared poll channels.
• PubSub package — lightweight internal/pubsub abstraction (Redis or NATS) used for shared poll cross-node notifications.

PostgreSQL MapBroker

postgres option for map_broker.type that implements the full MapBroker interface backed by PostgreSQL.

• Outbox-based delivery — writes go into a transactional outbox table; a delivery worker fans out to subscribers with configurable poll interval and batch size. Optional LISTEN/NOTIFY for low-latency wakeup.
• Sharded parallelism — channels are distributed across configurable shards for parallel delivery workers.
• Automatic schema management — EnsureSchema() creates tables, indexes, and functions on startup. Supports forward migrations via integer versioning. Can be disabled with skip_schema_init.
• JSONB or BYTEA — binary_data flag switches between JSONB (queryable) and BYTEA (binary/protobuf payloads) column types.
• TTL and cleanup — background workers handle key expiry and stream/meta/idempotency cleanup.