style: add explicit lifetime parameter to methods (64bit#450)

with changes to rust lifetime elision rules, the compiler
generates a warning for methods that return a type that
has a lifetime parameter tied to &self but are not explicit
about it

(cherry picked from commit 6becdfc)

Author: moldhouse

async-openai/src/embedding.rs

@@ -64,7 +64,6 @@ impl<'c, C: Config> Embeddings<'c, C> {
 
 #[cfg(test)]
 mod tests {
-    use crate::error::OpenAIError;
     use crate::types::{CreateEmbeddingResponse, Embedding, EncodingFormat};
     use crate::{Client, types::CreateEmbeddingRequestArgs};