feat(meta): make blocking sink default + support background sink

[kwannoel]

I hereby agree to the terms of the RisingWave Labs, Inc. Contributor License Agreement.

What's changed and what's your intention?

Part of #15587

1. Change sink to block by default again, until backfill completes.
2. If background ddl set, then we let create sink immediately succeed.

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.)

• 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)

Release note

This PR contains user-facing-changes to the behaviour of sink creation.
After this PR, we will block until backfill completes.
Before this PR, we will immediately let sink finish creating, once first barrier completes. After this PR, if this is the desired behaviour, set BACKGROUND_DDL=true.

[kwannoel]

Bump, seems like this PR was forgotten

[BugenZhao]

Rest LGTM

[BugenZhao]

Is this path still necessary? Can we now simply treat sinks like materialized views?

[kwannoel]

I suppose so. But it will require more refactoring of code.

Sink is unique from materialized view in that the output will immediately be visible, since it's external to the stream job.
For materialized views, we have to do something extra, which is to hide the output until backfill is complete.

As such we persist materialized views twice, when they undergo background ddl:

1. On initial creation. (mark as Creating, i.e. invisible, but recoverable).
2. After creation succeeds (mark as Finished, i.e. visible).

For sink we can immediately mark it as Finished (i.e. just step 2), we can't do that for MVs.

For Index, we have to also go through the same flow as materialized view. When I support Index I will also review to see if Sink can be supported likewise and unify more branches of code.

For now I think this is acceptable approach which requires least changes, and with simple concept.

[BugenZhao]

For sink we can immediately mark it as Finished (i.e. just step 2), we can't do that for MVs.

This is interesting to realize. 😄