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/document_loaders/web_base.py

@@ -1,4 +1,5 @@
 """Web base loader class."""
+
 import asyncio
 import logging
 import warnings
@@ -9,11 +10,12 @@ import requests
 from langchain_core.documents import Document
 
 from langchain_community.document_loaders.base import BaseLoader
+from langchain_community.utils.user_agent import get_user_agent
 
 logger = logging.getLogger(__name__)
 
 default_header_template = {
-    "User-Agent": "",
+    "User-Agent": get_user_agent(),
     "Accept": "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*"
     ";q=0.8",
     "Accept-Language": "en-US,en;q=0.5",