Add support for _meta attributes in resource contents

[calvingiles]

This change enables MCP servers to include metadata in resource content using the _meta field, which is part of the MCP specification. This allows servers to provide additional context about resources, such as domain information or other custom metadata.

Changes:

• Added meta field to ReadResourceContents helper class
• Added _meta field to Resource base class with proper alias
• Updated lowlevel server handler to pass through _meta when creating TextResourceContents and BlobResourceContents
• Updated FastMCP server to pass resource._meta to ReadResourceContents
• Added comprehensive tests for _meta support in both lowlevel and FastMCP resources

The implementation maintains backward compatibility - resources without _meta continue to work as before with meta=None.

Motivation and Context

How Has This Been Tested?

Breaking Changes

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Documentation update

Checklist

• [ ] I have read the MCP Documentation
• [ ] My code follows the repository's style guidelines
• [ ] New and existing tests pass locally
• [ ] I have added appropriate error handling
• [ ] I have added or updated documentation as needed

Additional context

[maxisbey]

related: https://github.com/modelcontextprotocol/python-sdk/pull/1476

[serranoarevalo]

Before this is merged, is there any workaround?