add singleflight for DescribeInstanceTypes

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.

Project coverage is 27.54%. Comparing base (94795f0) to head (89414ed).

Files	Patch %	Lines
pkg/aliyun/client/limit.go	0.00%	6 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #659 +/- ## ========================================== - Coverage 27.60% 27.54% -0.06% ========================================== Files 71 71 Lines 7879 7884 +5 ========================================== - Hits 2175 2172 -3 - Misses 5402 5409 +7 - Partials 302 303 +1 ``` | [Flag](https://app.codecov.io/gh/AliyunContainerService/terway/pull/659/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=AliyunContainerService) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/AliyunContainerService/terway/pull/659/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=AliyunContainerService) | `27.54% <0.00%> (-0.06%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=AliyunContainerService#carryforward-flags-in-the-pull-request-comment) to find out more.

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