[Bug]: The default config for FirestoreV1.write().batchWrite() on smaller java jobs has an abysmal throughput

### What happened?

Firestore has an 500 write/sec limitation, the java implementation splits up that quota among workers. Although it has a write rampup, the throughput is being influenced by the intial value a lot. The problem is that the default split is 1/500. So smaller jobs with just a few workers, start with 1/s and ramp up very slowly. If the job isn't sufficiently big enough, the throughtput stays low through the whole execution.

https://github.com/apache/beam/blob/69cd6ef06b377050a1289df011c5b76bb8b5c2d9/sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/firestore/RpcQosImpl.java#L119-L120

The note is there that it should be set, but you can easily miss it:
https://github.com/apache/beam/blob/69cd6ef06b377050a1289df011c5b76bb8b5c2d9/sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/firestore/RpcQosOptions.java#L258-L275

https://github.com/apache/beam/blob/69cd6ef06b377050a1289df011c5b76bb8b5c2d9/sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/firestore/RpcQosImpl.java#L594-L680

IMO the 500 as the default value should be decreased to a sensible much smaller one.

Also due to how the check is implemented at the write DoFn even that 1 batch write / second doesn't have the maximum size it could have as it doesn't collect the new entries while "throttling" it's just sitting doing nothing. So instead of collecting messages until limit + blocking till throttling, it just collects some (many cases 1) message and blocks for the throttling.

I would suggest that the blocking should only happen when the write is ready to be executed.
https://github.com/apache/beam/blob/69cd6ef06b377050a1289df011c5b76bb8b5c2d9/sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/firestore/FirestoreV1WriteFn.java#L293-L325

### Issue Priority

Priority: 2 (default / most bugs should be filed as P2)

### Issue Components

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Start with RpcQosImpl.java and RpcQosOptions.java, then trace the throttling path in FirestoreV1WriteFn.java. Compare the default quota split with the batching behavior described in the issue. Done means small jobs receive a sensible default throughput and batching waits at execution rather than idling before collecting entries.

Written by the indexing model from the issue text.