[PIP] Add `BatchReceive` to `Consumer`

**Is your feature request related to a problem? Please describe.**

The Java client exposes a batched receive mode on every `Consumer`:
```java
Messages msgs = consumer.batchReceive();
CompletableFuture> f = consumer.batchReceiveAsync();
```

Behaviour is governed by `BatchReceivePolicy`, configured on the builder via `consumerBuilder.batchReceivePolicy(...)`. The policy lets the user say "give me up to N messages, or up to S bytes, or wait at most T, whichever happens first."

Here the `Consumer` interface only offers single-message receive:
```go
Receive(context.Context) (Message, error)
```

There is no equivalent of `batchReceive`. Users who want batched processing have to loop over `Receive` themselves, but that approach:

- Cannot honour a "max bytes" boundary without re-implementing accounting.
- Cannot signal "I want partial batches if the timeout fires" cleanly.
- Forces an extra goroutine if you want to bound wait time independently of the per-message context deadline.

Batched receive is a major feature for high-throughput sinks: connectors, ETL processors, anything that wants to amortize a downstream write across many messages. It is one of the most-requested features when teams compare the Go client against the Java client.

**Describe the solution you'd like**

No breaking change. Add a `BatchReceivePolicy` and two methods on the `Consumer` interface. To stay compatible with existing implementations, expose the new methods with default no-op implementations on the multi-topic consumer first and roll out per-impl support.

**Describe alternatives you've considered**

## Edge cases for testing

- The first message arrives, then the channel becomes empty for longer than `Timeout` — return the partial batch.
- A `MaxNumBytes` of 1 with a payload of 100 — should return that one message, not get stuck.
- `BatchReceive` on a multi-topic consumer must round-robin or merge fairly, not starve some topics.
- Cancelling the parent `ctx` mid-batch should return what we already have, plus the context error.

**Additional context**

- Java: `pulsar-client-api/src/main/java/org/apache/pulsar/client/api/Consumer.java` (`batchReceive`, `batchReceiveAsync`)
- Java: `pulsar-client-api/src/main/java/org/apache/pulsar/client/api/BatchReceivePolicy.java`

**Are you willing to submit a PR?**

- [x] I'm willing to submit a PR!