feat: make Nats jetstream Source work in parallel

[tabVersion]

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What's changed and what's your intention?

tracked by #18876

Checklist

• I have written necessary rustdoc comments
• I have added necessary unit tests and integration tests
• I have added test labels as necessary. See details.
• I have added fuzzing tests or opened an issue to track them. (Optional, recommended for new SQL features Sqlsmith: Sql feature generation #7934).
• My PR contains breaking changes. (If it deprecates some features, please create a tracking issue to remove them in the future).
• All checks passed in ./risedev check (or alias, ./risedev c)
• My PR changes performance-critical code. (Please run macro/micro-benchmarks and show the results.)

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[tabVersion]

perf test using EC2

setup:

• Nats JS on EC2_1 (5,000,000 messages, file storage, GP3)
• RisingWave on EC2_2

---

test against main (b4bca57)

• ack all

create table t ( a int ) with ( connector = 'nats', server_url='172.31.46.172:4222', subject='testsubject', connect_mode='plain', consumer.durable_name = 'main_all_3', consumer.ack_policy = 'all', stream='teststream', consumer.max_ack_pending = '1000000') format plain encode json;

• ack explicit

create table t ( a int ) with ( connector = 'nats', server_url='172.31.46.172:4222', subject='testsubject', connect_mode='plain', consumer.durable_name = 'main_explicit_4', consumer.ack_policy = 'explicit', stream='teststream', consumer.max_ack_pending = '1000000') format plain encode json;

• ack none

create table t ( a int ) with ( connector = 'nats', server_url='172.31.46.172:4222', subject='testsubject', connect_mode='plain', consumer.durable_name = 'main_none_4', consumer.ack_policy = 'none', stream='teststream', consumer.max_ack_pending = '1000000') format plain encode json;

---

• 4 parallelism + ack all

set streaming_parallelism to 4;
create table t ( a int ) with ( connector = 'nats', server_url='172.31.46.172:4222', subject='testsubject', connect_mode='plain', consumer.durable_name = 'branch_all_3', consumer.ack_policy = 'all', stream='teststream', consumer.max_ack_pending = '1000000') format plain encode json;

• 4 parallelism + ack explicit

set streaming_parallelism to 4;
create table t ( a int ) with ( connector = 'nats', server_url='172.31.46.172:4222', subject='testsubject', connect_mode='plain', consumer.durable_name = 'branch_explicit_4', consumer.ack_policy = 'explicit', stream='teststream', consumer.max_ack_pending = '1000000') format plain encode json;

• 4 parallelism + ack none

set streaming_parallelism to 4;
create table t ( a int ) with ( connector = 'nats', server_url='172.31.46.172:4222', subject='testsubject', connect_mode='plain', consumer.durable_name = 'branch_none_3', consumer.ack_policy = 'none', stream='teststream', consumer.max_ack_pending = '1000000') format plain encode json;

[shanicky]

Should we assert that sources with adaptive enabled must also have scale-in enabled?

[fuyufjh]

How about renaming enable_scale_in to enable_drop_splits?