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]

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.

❗ 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 ⚠️
internal/merger/internal/aggregatemerger/merger.go	90.32%	2 Missing and 1 partial ⚠️
...merger/internal/groupbymerger/aggregator_merger.go	88.00%	2 Missing and 1 partial ⚠️
internal/merger/internal/sortmerger/merger.go	75.00%	2 Missing and 1 partial ⚠️
.../merger/internal/aggregatemerger/aggregator/avg.go	66.66%	2 Missing ⚠️
...erger/internal/aggregatemerger/aggregator/count.go	50.00%	2 Missing ⚠️
.../merger/internal/aggregatemerger/aggregator/min.go	50.00%	2 Missing ⚠️
.../merger/internal/aggregatemerger/aggregator/sum.go	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            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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