fix(nats): align offset and property fields between NATS and others for better ux and dx

[stdrc]

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What's changed and what's your intention?

Seems we now prefer the following style of connector properties:

scan.startup.mode='timestamp',
scan.startup.timestamp.millis='123456'

This PR refines the NATS source connector to align with this style.

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).
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• My PR changes performance-critical code. (Please run macro/micro-benchmarks and show the results.)

• My PR contains critical fixes that are necessary to be merged into the latest release. (Please check out the details)

Documentation

• My PR needs documentation updates. (Please use the Release note section below to summarize the impact on users)

risingwavelabs/risingwave-docs#2643

[stdrc]

• fix(nats): align the semantics of scan.startup.mode=latest of NATS with others #18733
• fix(nats): align offset and property fields between NATS and others for better ux and dx #18732 👈
• main

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[yufansong]

LGTM