community[minor]: add user agent for web scraping loaders (#22480)

**Description:** This PR adds a `USER_AGENT` env variable that is to be
used for web scraping. It creates a util to get that user agent and uses
it in the classes used for scraping in [this piece of
doc](https://python.langchain.com/v0.1/docs/use_cases/web_scraping/).
Identifying your scraper is considered a good politeness practice, this
PR aims at easing it.
**Issue:** `None`
**Dependencies:** `None`
**Twitter handle:** `None`

libs/community/langchain_community/utils/user_agent.py

@@ -0,0 +1,16 @@
+import logging
+import os
+
+log = logging.getLogger(__name__)
+
+
+def get_user_agent() -> str:
+    """Get user agent from environment variable."""
+    env_user_agent = os.environ.get("USER_AGENT")
+    if not env_user_agent:
+        logging.warning(
+            "USER_AGENT environment variable not set, "
+            "consider setting it to identify your requests."
+        )
+        return "DefaultLangchainUserAgent"
+    return env_user_agent