Support metadata and output format in custom tools #2718

Currently, the custom tools can only return a "string," which poses an issue for tools that need to return markdown and extra metadata.

I used a forked version of opencode that had my custom tool support including improved output display for custom tools.
I am happy to submit a PR for this.

Furthermore, I have a command line "tools" command that can list tools and execute them, which is useful when making custom tools and configuring agents. I am happy to add this feature to the project if anyone else would find it useful.