added system prompt and template fields to ollama (#13022)

**Description**
the ollama api now supports passing system prompt and template directly
instead of modifying the model file , but the ollama integration in
langchain did not have this change updated . The update just adds these
two parameters to it ( there are 2 more parameters that are pending to
be updated, I was not sure about their utility wrt to langchain )
Refer :
8713ac23a8

**Issue** : None Applicable

**Dependencies** : None Changed

**Twitter handle** : https://twitter.com/violetto96

---------

Co-authored-by: Eugene Yurtsev <eyurtsev@gmail.com>

libs/langchain/langchain/llms/ollama.py

@@ -89,6 +89,12 @@ class _OllamaCommon(BaseLanguageModel):
     to more diverse text, while a lower value (e.g., 0.5) will
     generate more focused and conservative text. (Default: 0.9)"""
 
+    system: Optional[str] = None
+    """system prompt (overrides what is defined in the Modelfile)"""
+
+    template: Optional[str] = None
+    """full prompt or prompt template (overrides what is defined in the Modelfile)"""
+
     @property
     def _default_params(self) -> Dict[str, Any]:
         """Get the default parameters for calling Ollama."""
@@ -109,6 +115,8 @@ class _OllamaCommon(BaseLanguageModel):
                 "top_k": self.top_k,
                 "top_p": self.top_p,
             },
+            "system": self.system,
+            "template": self.template,
         }
 
     @property