feat(suscription): Improving usability of subscription

[xxhZs]

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

What's changed and what's your intention?

#18208

1. Change the default behavior of DECLARE cursor_name SUBSCRIPTION CURSOR FOR subscription_name to SINCE now() without backfilling historical data.
2. Change op column type from int16 to varchar to make it more understandable: 1 -> insert, 2 -> update_insert, 3 -> delete, 4 -> update_delete
3. support show cursors and show subscription cursors:
   show cursors will return all query cursor in this seesion
   show subscription cursors will return all subscription cursor and subscription name in this seesion

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

after this pr

1. BREAKING CHANGE: DECLARE cursor_name SUBSCRIPTION CURSOR = DECLARE cursor_name SUBSCRIPTION CURSOR since now(), will be consumed from the current time. DECLARE cursor_name SUBSCRIPTION CURSOR FULL， Will start consuming data from stock
2. BREAKING CHANGE: The type of op has changed to varchar，It's one of Insert Delete UpdateInset UpdateDelete.
3. show cursors will return all query cursor in this seesion
   show subscription cursors will return all subscription cursor and subscription name in this seesion

[hzxa21]

Do we need to modify the cursor test as well?

Rest LGTM!

[hzxa21]

I think it is more useful to include the following information as well:

• seek_pos: the rw_timestamp or the the seek mode (FULL/BEGIN/NOW/...) when the cursor is declare
• num_rows_fetched: number of rows fetched internally by subscription cursor
• num_rows_returned: number of rows returned to user

[hzxa21]

We can do this in a separate PR as well to speed up merging this PR with breaking changes to 2.0.