feat(bigtable): make write batch size and concurrency configurable

[Reactor11]

What this PR does / why we need it

The Bigtable online store hardcodes two values that govern its write path:

• MUTATIONS_PER_OP = 50_000 — target mutations per MutateRows request
• BIGTABLE_CLIENT_CONNECTION_POOL_SIZE = 10 — the ThreadPoolExecutor size used to parallelize writes in online_write_batch

On a shared Bigtable instance this is a problem: a large materialization fans its writes out across the thread pool with no way to tune the request size or concurrency, issuing an unthrottled write burst that can saturate the instance and inflate read-path tail latency for other workloads sharing it. Today the only way to soften that burst is to fork the online store.

This PR exposes both as optional BigtableOnlineStoreConfig fields:

Field	Default	Controls
mutations_per_write	50000	mutations per Bigtable write request
write_concurrency	10	concurrent write requests (thread-pool size) per worker

Both default to the existing module constants, so behavior is unchanged unless explicitly configured. Operators running against a shared instance can now lower either value to reduce the write load a materialization places on Bigtable, trading materialization speed for lower peak write pressure. Both are validated as PositiveInt, and online_write_batch now floors rows-per-request at 1 so a very wide feature view combined with a small mutations_per_write can't produce a zero-sized batch.

Example:

online_store:
  type: bigtable
  instance: my-instance
  mutations_per_write: 10000
  write_concurrency: 2

Which issue(s) this PR fixes

N/A — backward-compatible enhancement.

Misc

• Adds sdk/python/tests/unit/infra/online_store/test_bigtable_online_store.py covering defaults matching the legacy constants, positive-int validation, batch chunking, the wide-feature-view floor, and the configurable thread-pool size.

[Reactor11]

Hi @franciscojavierarceo please review this.

At our org, we want to control the number of writes and concurrency. As of now it is hard coded.

[shuchu]

lgtm