Support normal-topic consumption with the gRPC SimpleConsumer

[lizhimins]

Search before asking

• I had searched in the issues and found no similar issues.

Motivation

The gRPC connector added in #135 / #136 only supports the LITE consumption model: the source binds a
main lite topic through LiteSimpleConsumer and defers acknowledgement to a downstream operator,
which is what enables fair per-sub-topic throttling.

That leaves the most common case uncovered: consuming a plain normal topic over gRPC. Users who
just want topic + tag/SQL92 filter today have to either fall back to the remoting connector or
model their topic as a lite topic, and they inherit a downstream ack operator they do not need.

Solution

Add a SIMPLE consumption mode alongside LITE, selected by a single rocketmq.source.mode
(SQL: source.mode) option:

• Subscribe one normal topic through the SDK SimpleConsumer with an optional tag / SQL92 filter
  expression.
• Acknowledge on the source side, aligned with checkpoints — the same approach the GCP Pub/Sub
  connector uses: receipt handles of emitted records are grouped per checkpoint and acked from
  notifyCheckpointComplete, giving at-least-once. Un-acked messages are redelivered once their
  invisible duration expires.
• Issue those acks through the very consumer that received the messages, so no second gRPC
  connection and no downstream ack operator are involved.
• Abstract the two SDK Pop clients (SimpleConsumer / LiteSimpleConsumer) behind one internal
  interface, since they expose the same receive / ack / changeInvisibleDuration operations but share
  no common type.
• Expose the mode and the filter options through the SQL connector as well.

LITE remains fully supported for the lite-topic + downstream-throttling scenario.

Are you willing to submit a PR?

• I'm willing to submit a PR!