community : allow using apikey for PubMedAPIWrapper (#27246)

**Description**: 

> Without an API key, any site (IP address) posting more than 3 requests
per second to the E-utilities will receive an error message. By
including an API key, a site can post up to 10 requests per second by
default.

quoted from A General Introduction to the E-utilities,NCBI :
https://www.ncbi.nlm.nih.gov/books/NBK25497/

I have simply added a api_key parameter to the PubMedAPIWrapper that can
be used to increase the number of requests per second from 3 to 10.

**Twitter handle** : @KORmaori

---------

Co-authored-by: Erick Friis <erick@langchain.dev>

libs/community/langchain_community/utilities/pubmed.py

@@ -31,6 +31,7 @@ class PubMedAPIWrapper(BaseModel):
         sleep_time: time to wait between retries.
           Default is 0.2 seconds.
         email: email address to be used for the PubMed API.
+        api_key: API key to be used for the PubMed API.
     """
 
     parse: Any  #: :meta private:
@@ -47,6 +48,7 @@ class PubMedAPIWrapper(BaseModel):
     MAX_QUERY_LENGTH: int = 300
     doc_content_chars_max: int = 2000
     email: str = "your_email@example.com"
+    api_key: str = ""
 
     @model_validator(mode="before")
     @classmethod
@@ -101,6 +103,8 @@ class PubMedAPIWrapper(BaseModel):
             + str({urllib.parse.quote(query)})
             + f"&retmode=json&retmax={self.top_k_results}&usehistory=y"
         )
+        if self.api_key != "":
+            url += f"&api_key={self.api_key}"
         result = urllib.request.urlopen(url)
         text = result.read().decode("utf-8")
         json_text = json.loads(text)
@@ -135,6 +139,8 @@ class PubMedAPIWrapper(BaseModel):
             + "&webenv="
             + webenv
         )
+        if self.api_key != "":
+            url += f"&api_key={self.api_key}"
 
         retry = 0
         while True: