[langchain-sqlserver] Add support for upserting documents

[yorek]

When using add_texts or add_documents or from_documents, it should be possible to have an option named upsert that, if set to true will

• insert a new row in the underlying table if a row with the same id doesn't exists or,
• update an existing row with the new data, if a row with the same id is existing already

The id used to check if a row is existing or not is the id provided by the developer, currently saved in the custom_id column, By default upsert will be set to false to keep consistency with other vector stores.