Adds optional _meta field to Tool (#339)

<!-- Provide a brief summary of your changes -->

## Motivation and Context
The recently released openAI [Apps SDK
](https://developers.openai.com/apps-sdk/reference/#_meta-fields-the-client-provides)adds
a '__meta_' field on the Tool Descriptor. This would be in addition to
it being available on Messages. It remains optional and defaults to an
empty json object, following existing conventions.


## How Has This Been Tested?
Unit tests for serialization added

## Breaking Changes
N/A

## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [X] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update

## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
- [X] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [X] My code follows the repository's style guidelines
- [X] New and existing tests pass locally
- [X] I have added appropriate error handling
- [X] I have added or updated documentation as needed

## Additional context
https://developers.openai.com/apps-sdk

https://developers.openai.com/apps-sdk/reference/#_meta-fields-the-client-provides

---------

Co-authored-by: Pavel Gorgulov <devcrocod@gmail.com>

kotlin-sdk-client/src/jvmTest/kotlin/io/modelcontextprotocol/kotlin/sdk/client/StreamableHttpClientTest.kt

@@ -113,7 +113,8 @@ internal class StreamableHttpClientTest : AbstractStreamableHttpClientTest() {
                         }
                       },
                       "required": ["temperature"]
-                    }
+                    },
+                    "_meta": {}
                   }
                 ]
               }