fix(go): Made WithTools accept a ToolRef interface

[apascal07]

ai.WithTools took in a Tool which is a resolved tool action. This works for in-code usages but when loading .prompt files, we only have tool names as strings. We can resolve these but since we load .prompt files at Genkit init, tools won't have been initialized yet and during prompt definition, we convert the tools to tool names to adhere to the GenerateActionOptions interface anyway. But the user experience is still nicer when defining tools and passing those tool references directly so this is the compromise.

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)