feat(streaming): reduce join cache entry size

[yuhao-su]

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

What's changed and what's your intention?

Use Vec instead of BTreeMap when the number of elements is small to save memory usage.

Checklist

• I have written necessary rustdoc comments
• I have added necessary unit tests and integration tests
• 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

If this PR includes changes that directly affect users or other significant modifications relevant to the community, kindly draft a release note to provide a concise summary of these changes. Please prioritize highlighting the impact these changes will have on users.

[codecov]

Codecov Report

Merging #13214 (a856beb) into main (73dcbc2) will decrease coverage by 0.08%.
Report is 10 commits behind head on main.
The diff coverage is 71.60%.

@@            Coverage Diff             @@
##             main   #13214      +/-   ##
==========================================
- Coverage   68.14%   68.07%   -0.08%     
==========================================
  Files        1513     1516       +3     
  Lines      256990   257175     +185     
==========================================
- Hits       175135   175069      -66     
- Misses      81855    82106     +251     

Flag	Coverage Δ
rust	68.07% <71.60%> (-0.08%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
src/meta/src/backup_restore/utils.rs	43.58% <ø> (ø)
src/storage/backup/src/storage.rs	63.55% <100.00%> (+0.34%)	⬆️
src/storage/src/hummock/backup_reader.rs	31.17% <ø> (ø)
...rc/executor/managed_state/join/join_entry_state.rs	98.91% <100.00%> (+0.52%)	⬆️
src/stream/src/executor/managed_state/join/mod.rs	93.79% <ø> (ø)
src/storage/backup/src/lib.rs	56.81% <66.66%> (-0.33%)	⬇️
...am/src/executor/managed_state/join/join_row_set.rs	75.92% <75.92%> (ø)
src/meta/src/backup_restore/restore.rs	83.33% <63.15%> (-2.45%)	⬇️
src/meta/src/backup_restore/restore_impl/v1.rs	87.87% <87.87%> (ø)
src/meta/src/backup_restore/restore_impl/v2.rs	0.00% <0.00%> (ø)

... and 17 files with indirect coverage changes

📣 Codecov offers a browser extension for seamless coverage viewing on GitHub. Try it in Chrome or Firefox today!

[stdrc]

LGTM