Add configurable gRPC keepalive for Storage Write API streams

When using the BigQuery Storage Write API (`useStorageWriteApi: true`), gRPC streams are silently closed by Google's server-side infrastructure after ~600 seconds of inactivity. The next `AppendRows` call on the dead stream fails with:

ABORTED: Closing the stream because it has been inactive for 600 seconds

or:

io.grpc.StatusRuntimeException: UNAVAILABLE: Connection reset by peer

This triggers task failures and automatic restarts. In low-traffic connectors (e.g., topics with variable producer rates), this creates a **flapping pattern** — tasks repeatedly fail and restart during idle windows, causing:

- Temporary data delivery delays during stream recovery
- Downstream measurement gaps (consumers expecting continuous flow see holes)
- Log noise that masks real failures

There is no way to configure gRPC keepalive via connector config, JVM system properties, or environment variables. The `BigQueryWriteSettings` builder in `StorageWriteApiBase` does not expose transport channel options.

## Root Cause

In `StorageWriteApiBase.java`, the `BigQueryWriteSettings` is constructed without any transport-level configuration:

```java
BigQueryWriteSettings.newBuilder()
.setCredentialsProvider(FixedCredentialsProvider.create(credentials))
.build();

Start by reading StorageWriteApiBase.java, especially the BigQueryWriteSettings builder, and trace how useStorageWriteApi is configured. Determine how connector configuration should reach the transport channel options and identify the existing tests covering Storage Write API streams. Done means low-traffic streams can use a configured gRPC keepalive and no longer fail solely after the server's idle period.

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