Normalize `Callable` naming convention

[vasinov]

Multiple Griptape classes have Callable properties. We use the following naming convention for them:

1. _func. For example SnowflakeSqlDriver.connection_func.
2. _fn. For example, LocalVectorStoreDriver.relatedness_fn.
3. Method-like. For example, BasePromptDriver.prompt_stack_to_string.

The team decided to normalize all callable properties to follow the _fn naming convention.