This PR extends upon @jzluo 's PR #2748 which addressed dialect-specific
issues with SQL prompts, and adds a prompt that uses backticks for
column names when querying BigQuery. See [GoogleSQL quoted
identifiers](https://cloud.google.com/bigquery/docs/reference/standard-sql/lexical#quoted_identifiers).
Additionally, the SQL agent currently uses a generic prompt. Not sure
how best to adopt the same optional dialect-specific prompts as above,
but will consider making an issue and PR for that too. See
[langchain/agents/agent_toolkits/sql/prompt.py](langchain/agents/agent_toolkits/sql/prompt.py).
_googlesql_prompt="""You are a GoogleSQL expert. Given an input question, first create a syntactically correct GoogleSQL query to run, then look at the results of the query and return the answer to the input question.
_mssql_prompt="""You are an MS SQL expert. Given an input question, first create a syntactically correct MS SQL query to run, then look at the results of the query and return the answer to the input question.