community[patch]: OllamaEmbeddings - Pass headers to post request (#16880)

## Feature
- Set additional headers in constructor
- Headers will be sent in post request

This feature is useful if deploying Ollama on a cloud service such as
hugging face, which requires authentication tokens to be passed in the
request header.

## Tests
- Test if header is passed
- Test if header is not passed

Similar to https://github.com/langchain-ai/langchain/pull/15881

---------

Co-authored-by: Bagatur <baskaryan@gmail.com>

libs/community/tests/unit_tests/llms/test_ollama.py

@@ -25,7 +25,7 @@ def test_pass_headers_if_provided(monkeypatch: MonkeyPatch) -> None:
         base_url="https://ollama-hostname:8000",
         model="foo",
         headers={
-            "Authentication": "Bearer TEST-TOKEN-VALUE",
+            "Authorization": "Bearer TEST-TOKEN-VALUE",
             "Referer": "https://application-host",
         },
         timeout=300,
@@ -35,7 +35,7 @@ def test_pass_headers_if_provided(monkeypatch: MonkeyPatch) -> None:
         assert url == "https://ollama-hostname:8000/api/generate"
         assert headers == {
             "Content-Type": "application/json",
-            "Authentication": "Bearer TEST-TOKEN-VALUE",
+            "Authorization": "Bearer TEST-TOKEN-VALUE",
             "Referer": "https://application-host",
         }
         assert json is not None