Missing _meta Field Support in rmcp Tool Registration

[Kelach]

Hello!

I am trying to build a ChatGPT App using the official rmcp Rust MCP SDK.
According to the OpenAI Apps SDK requirements, each tool needs to provide a _meta field at registration (for example, to specify openai/outputTemplate for UI component integration).

However, the rmcp::model::Tool struct does not seem to support a _meta or similar metadata field at registration time — making it impossible to declare required OpenAI metadata for tool-based widget rendering.

To Reproduce

Steps to reproduce the behavior:

1. Attempt to define/register a tool using rmcp with a _meta (or any OpenAI-specific metadata, like openai/outputTemplate).
2. Notice there is no supported field for this in the tool struct or macro.

Expected Behavior

It should be possible to attach OpenAI-specific metadata (such as _meta or openai/outputTemplate) to tools during registration.

Maybe I'm missing context, but it seems there's a lack of support for a _meta field on the Tool struct.
Please consider adding support for it — I'm happy to draft a PR to do so.

Ideally, this new field is treated as a general-purpose JSON object, so additional per-tool metadata can be supplied during registration.