feat(frontend): change streaming_rate_limit=0 to pause stream instead of disable rate limit

[kwannoel]

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What's changed and what's your intention?

Support rate limit 0. Its behaviour is to pause the backfill / source stream.

Subsequent PR will add ALTER MV with (enable_streaming_rate_limit=false), so we can also test it in deterministic tests.

To disable rate limit use streaming_rate_limit=default instead.

Checklist

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Documentation

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Release note

1. set streaming_rate_limit=0 now means pausing the snapshot read stream for backfill, pausing source read for sources. It previously disabled rate limit within the session.
2. set streaming_rate_limit=default will disable rate limit within the session. It does not alter existing DDLs' rate limits.

[tabVersion]

thanks, I forgot there is still some work to do in the frontend.

[fuyufjh]

LGTM