feat: add LazyValue::as_cow_str

[utkarshgupta137]

What type of PR is this?

feat

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More detailed description for this PR.

Add LazyValue::as_cow_str to support zero-copy get.

Currently, sonic-rs::get returns LazyValue, but LazyValue::as_str returns &str which is tied to the lifetime of the LazyValue reference instead of the input. This means that even if there are no escapes, we've to copy the str to return the value. LazyValue::as_cow_str returns a Cow<'a, str> which is tied to the lifetime of the input.

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[liuq19]

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Codecov Report

❌ Patch coverage is 0% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.87%. Comparing base (32668d6) to head (1a2ee6c).

Files with missing lines	Patch %	Lines
src/lazyvalue/value.rs	0.00%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #217      +/-   ##
==========================================
- Coverage   69.97%   69.87%   -0.10%     
==========================================
  Files          42       42              
  Lines        9917     9930      +13     
==========================================
  Hits         6939     6939              
- Misses       2978     2991      +13     

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Codex Review: Didn't find any major issues. Bravo.

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