refactor(batch): easier for writing tests with customized exchange source

[lmatz]

What's changed and what's your intention?

Use a concrete struct to create a source instead of merely generics.

For each concrete source, we have a dedicated concrete source creator struct to initialize. This is helpful when we want to ingest different data into different sources during tests.

Checklist

- [ ] I have written necessary docs and comments
- [ ] I have added necessary unit tests and integration tests

• All checks passed in ./risedev check (or alias, ./risedev c)

Refer to a related PR or issue link (optional)

closes #1645

[codecov]

Codecov Report

Merging #3236 (b1e308d) into main (47fb009) will decrease coverage by 0.01%.
The diff coverage is 88.00%.

@@            Coverage Diff             @@
##             main    #3236      +/-   ##
==========================================
- Coverage   73.41%   73.40%   -0.02%     
==========================================
  Files         744      744              
  Lines      101665   101644      -21     
==========================================
- Hits        74634    74608      -26     
- Misses      27031    27036       +5     

Flag	Coverage Δ
rust	73.40% <88.00%> (-0.02%)	⬇️

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

Impacted Files	Coverage Δ
src/batch/src/executor/generic_exchange.rs	47.57% <77.77%> (-11.96%)	⬇️
src/batch/src/executor/merge_sort_exchange.rs	67.62% <80.00%> (-4.71%)	⬇️
src/batch/src/executor/test_utils.rs	94.94% <100.00%> (+1.44%)	⬆️
src/meta/src/hummock/mock_hummock_meta_client.rs	42.85% <0.00%> (-1.10%)	⬇️
src/storage/src/hummock/local_version_manager.rs	83.86% <0.00%> (-0.16%)	⬇️
src/meta/src/barrier/mod.rs	69.45% <0.00%> (+0.32%)	⬆️

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[xxchan]

LGTM