Add `Client.close()` and context manager

Currently, a client to interact with the Ollama API can be created using

from ollama import Client
client = Client(host='http://localhost:11434', ...)

However, there is no way to close the client and give back the resources it acquired. Eventually, Python's garbage collector will, but it would be nice to be able to do that as early as possible.

From my understanding, adding the following methods to ollama._client.Client would address this:

class Client(BaseClient):
    ...
    def close(self):
        self._client.close()

    def __enter__(self):
        return self

    def __exit__(self, exc_type, exc_value, traceback):
        self.close()
        return False

And then something similar for the AsyncClient.

That way, the client could be used like:

with Client(host='http://localhost:11434', ...) as client:
    client.chat(...)

If you want, I can look into this and prepare a PR.

Update: This should also fix warnings when using ollama-python with pytest:

.../.venv/lib/python3.12/site-packages/_pytest/unraisableexception.py:31: ResourceWarning: unclosed <socket.socket fd=15, family=2, type=1, proto=6, laddr=('127.0.0.1', 64484), raddr=('127.0.0.1', 11434)>
gc.collect()

Start by reading ollama._client.Client and AsyncClient, including how each underlying _client is created. Verify that close and context-manager behavior releases resources for both clients and addresses the reported pytest ResourceWarning.