[SPARK-44312][CONNECT][PYTHON] Allow to set a user agent with an envi…

…ronment variable

### What changes were proposed in this pull request?
Use the `SPARK_CONNECT_USER_AGENT` environment variable as a fallback for the prepended user agent string if one did not set the user agent in `ChannelBuilder`.

### Why are the changes needed?
Currently one has to specify a custom user agent string in `ChannelBuilder`. It would be useful to be able to set this string with an environment variable.

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?
Manual testing + existing tests.

Closes apache#41866 from dillitz/SPARK-44312-user-agent-environment.

Authored-by: Robert Dillitz <robert.dillitz@databricks.com>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>

python/pyspark/sql/connect/client/core.py

@@ -296,7 +296,10 @@ def userAgent(self) -> str:
             or "_SPARK_CONNECT_PYTHON" when not specified.
             The returned value will be percent encoded.
         """
-        user_agent = self.params.get(ChannelBuilder.PARAM_USER_AGENT, "_SPARK_CONNECT_PYTHON")
+        user_agent = self.params.get(
+            ChannelBuilder.PARAM_USER_AGENT,
+            os.getenv("SPARK_CONNECT_USER_AGENT", "_SPARK_CONNECT_PYTHON"),
+        )
         ua_len = len(urllib.parse.quote(user_agent))
         if ua_len > 2048:
             raise SparkConnectException(