Bulk publish: per-message metadata in publish_events

[MyMirelHub]

Describe the feature

publish_events does not populate BulkPublishRequestEntry.metadata, so per-message metadata (partition key, session id, Kafka key) cannot be set. Entries are built from entry_id, event and content_type only, in both dapr/clients/grpc/client.py#L544 and dapr/aio/clients/grpc/client.py#L493.

The runtime supports it, and go-sdk (PublishEventsEvent.Metadata), java-sdk (BulkPublishEntry) and js-sdk (PubSubBulkPublishMessage.metadata) all expose it. Same request against dotnet-sdk in dapr/dotnet-sdk#1198.

Side effect: the runtime only merges request-level metadata into entries that already have metadata, so publish_metadata never reaches the envelope either. On 1.18.4 with ttlInSeconds, publish_event sets expiration on the cloudevent and publish_events does not.

Proposed API, mirroring go-sdk:

d.publish_events(
    pubsub_name='pubsub',
    topic_name='TOPIC_A',
    data=[
        BulkPublishEntry(event='{"n": 1}', metadata={'partitionKey': 'tenant-a'}),
        BulkPublishEntry(event='{"n": 2}', metadata={'partitionKey': 'tenant-b'}),
        '{"n": 3}',
    ],
)

Existing bytes/str entries keep working, and publish_metadata should be copied onto every entry.

Release Note

RELEASE NOTE: ADD Per-message metadata support in publish_events bulk publish.