add sqlalchemy generic cache (#361)

Created a generic SQLAlchemyCache class to plug any database supported
by SQAlchemy. (I am using Postgres).
I also based the class SQLiteCache class on this class SQLAlchemyCache.

As a side note, I'm questioning the need for two distinct class
LLMCache, FullLLMCache. Shouldn't we merge both ?

docs/examples/prompts/llm_functionality.ipynb

@@ -373,12 +373,19 @@
    "id": "6053408b",
    "metadata": {},
    "outputs": [],
-   "source": []
+   "source": [
+    "# You can use SQLAlchemyCache to cache with any SQL database supported by SQLAlchemy.\n",
+    "from langchain.cache import SQLAlchemyCache\n",
+    "from sqlalchemy import create_engine\n",
+    "\n",
+    "engine = create_engine(\"postgresql://postgres:postgres@localhost:5432/postgres\")\n",
+    "langchain.llm_cache = SQLAlchemyCache(engine)"
+   ]
   }
  ],
  "metadata": {
   "kernelspec": {
-   "display_name": "Python 3 (ipykernel)",
+   "display_name": "base",
    "language": "python",
    "name": "python3"
   },
@@ -392,7 +399,12 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.10.8"
+   "version": "3.9.12 (main, Jun  1 2022, 06:34:44) \n[Clang 12.0.0 ]"
+  },
+  "vscode": {
+   "interpreter": {
+    "hash": "1235b9b19e8e9828b5c1fdb2cd89fe8d3de0fcde5ef5f3db36e4b671adb8660f"
+   }
   }
  },
  "nbformat": 4,