community: update dynamodb chat history to update instead of overwrite

[Davi-Schumacher]

Description:
The current implementation of DynamoDBChatMessageHistory updates the History attribute for a given chat history record by first extracting the existing contents into memory, appending the new message, and then using the put_item method to put the record back. This has the effect of overwriting any additional attributes someone may want to include in the record, like chat session metadata.

This PR suggests changing from using put_item to using update_item instead which will keep any other attributes in the record untouched. The change is backward compatible since

1. update_item is an "upsert" operation, creating the record if it doesn't already exist, otherwise updating it
2. It only touches the db insert call and passes the exact same information. The rest of the class is left untouched

Dependencies:
None

Tests and docs:
No unit tests currently exist for the DynamoDBChatMessageHistory class. This PR adds the file libs/community/tests/unit_tests/chat_message_histories/test_dynamodb_chat_message_history.py to test the add_message and clear methods. I wanted to use the moto library to mock DynamoDB calls but I could not get poetry to resolve it so I mocked those calls myself in the test. Therefore, no test dependencies were added.

The change was tested on a test DynamoDB table as well. The first three images below show the current behavior. First a message is added to chat history, then a value is inserted in the record in some other attribute, and finally another message is added to the record, destroying the other attribute.

The next three images show the new behavior. Once again a value is added to an attribute other than the History attribute, but now when the followup message is added it does not destroy that other attribute. The History attribute itself is unaffected by this change.

The doc located at docs/docs/integrations/memory/aws_dynamodb.ipynb required no changes and was tested as well.

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