Add support for custom dependencies in tools

[vasinov]

BaseTool should have a dependencies property that can be used to add custom dependencies to tools. This is useful for tools like SqlClient.

[collindutter]

@vasinov In the case of SqlClient, we cannot install dependencies on tool instantiation because it requires a SqlDriver to create the tool. And the SqlDriver will throw an error if the necessary dependencies are not already installed i.e. psycopg2.