Automatically fill optional tool activity schema defaults if no values is provided

[vasinov]

Tool activities can use schema.Optional fields with defaults set by the tool creator. Today, tool creators have to manually check for the value passed by the LLM and then manually fill in the default in the activity implementation.

I propose adding automatic activity default propagation in ActionSubtask in addition to the already implemented activity input validation.

[collindutter]

FYI I started this work but it's non-trivial to extract default values from a schema.Schema instance. https://github.com/griptape-ai/griptape/issues/1499 may help here since we can at least hide the uglyness in a Driver specific to schema.