community[minor]: Adds Llamafile as an LLM (#17431)

* **Description:** Adds a simple LLM implementation for interacting with
[llamafile](https://github.com/Mozilla-Ocho/llamafile)-based models.
* **Dependencies:** N/A
* **Issue:** N/A

**Detail**
[llamafile](https://github.com/Mozilla-Ocho/llamafile) lets you run LLMs
locally from a single file on most computers without installing any
dependencies.

To use the llamafile LLM implementation, the user needs to:

1. Download a llamafile e.g.
https://huggingface.co/jartine/TinyLlama-1.1B-Chat-v1.0-GGUF/resolve/main/TinyLlama-1.1B-Chat-v1.0.Q5_K_M.llamafile?download=true
2. Make the file executable.
3. Run the llamafile in 'server mode'. (All llamafiles come packaged
with a lightweight server; by default, the server listens at
`http://localhost:8080`.)


```bash
wget https://url/of/model.llamafile
chmod +x model.llamafile
./model.llamafile --server --nobrowser
```

Now, the user can invoke the LLM via the LangChain client:

```python
from langchain_community.llms.llamafile import Llamafile

llm = Llamafile()

llm.invoke("Tell me a joke.")
```

libs/community/tests/integration_tests/llms/test_llamafile.py

@@ -0,0 +1,46 @@
+import os
+from typing import Generator
+
+import pytest
+import requests
+from requests.exceptions import ConnectionError, HTTPError
+
+from langchain_community.llms.llamafile import Llamafile
+
+LLAMAFILE_SERVER_BASE_URL = os.getenv(
+    "LLAMAFILE_SERVER_BASE_URL", "http://localhost:8080"
+)
+
+
+def _ping_llamafile_server() -> bool:
+    try:
+        response = requests.get(LLAMAFILE_SERVER_BASE_URL)
+        response.raise_for_status()
+    except (ConnectionError, HTTPError):
+        return False
+
+    return True
+
+
+@pytest.mark.skipif(
+    not _ping_llamafile_server(),
+    reason=f"unable to find llamafile server at {LLAMAFILE_SERVER_BASE_URL}, "
+    f"please start one and re-run this test",
+)
+def test_llamafile_call() -> None:
+    llm = Llamafile()
+    output = llm.invoke("Say foo:")
+    assert isinstance(output, str)
+
+
+@pytest.mark.skipif(
+    not _ping_llamafile_server(),
+    reason=f"unable to find llamafile server at {LLAMAFILE_SERVER_BASE_URL}, "
+    f"please start one and re-run this test",
+)
+def test_llamafile_streaming() -> None:
+    llm = Llamafile(streaming=True)
+    generator = llm.stream("Tell me about Roman dodecahedrons.")
+    assert isinstance(generator, Generator)
+    for token in generator:
+        assert isinstance(token, str)