refactor(merger): 重构AVG函数实现,重构所有rows.Rows实现的ConlumnType方法并添加测试

[longyue0521]

依赖于 #222 , 需要先合并 #222

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 81.25000% with 21 lines in your changes missing coverage. Please review.

Project coverage is 83.00%. Comparing base (dae71da) to head (e1940c0). Report is 3 commits behind head on dev.

:exclamation: Current head e1940c0 differs from pull request most recent head 88af252

Please upload reports for the commit 88af252 to get more accurate results.

Files	Patch %	Lines
.../merger/internal/aggregatemerger/aggregator/max.go	0.00%	4 Missing :warning:
internal/merger/internal/aggregatemerger/merger.go	90.32%	2 Missing and 1 partial :warning:
...merger/internal/groupbymerger/aggregator_merger.go	88.00%	2 Missing and 1 partial :warning:
internal/merger/internal/sortmerger/merger.go	75.00%	2 Missing and 1 partial :warning:
.../merger/internal/aggregatemerger/aggregator/avg.go	66.66%	2 Missing :warning:
...erger/internal/aggregatemerger/aggregator/count.go	50.00%	2 Missing :warning:
.../merger/internal/aggregatemerger/aggregator/min.go	50.00%	2 Missing :warning:
.../merger/internal/aggregatemerger/aggregator/sum.go	50.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #223 +/- ## ========================================== + Coverage 82.78% 83.00% +0.21% ========================================== Files 52 53 +1 Lines 3951 4242 +291 ========================================== + Hits 3271 3521 +250 - Misses 563 592 +29 - Partials 117 129 +12 ```

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