feat: add LazyValue::as_cow_str

[utkarshgupta137]

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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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