chore: Make SyncSpec and AsyncSpec extensible

[He-Pin]

Motivation and Context

We need to extend McpSpec to add more functions.

NOTE: I think we should try to make use of lombok's @superBuilder here, which can make our life easier, instead of a copy.

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

[Kehrlann]

We tend to prefer composition over inheritance. Please document what you are trying to achieve before proposing a change.

We are not planning to add Lombok to this codebase.

[He-Pin]

Thanks. In our current implementation, we need to return our own McpAsyncClient implementation (which we've partially customized to support our scenario), but we also want users to maintain their previous usage habits. Since we currently cannot extend McpClient's AsyncSpec (which has a private constructor), we have to copy the AsyncSpec code and make some modifications. Although the license allows us to do this, it's extremely inconvenient for maintainability. Therefore, we hope to have this extension so that users with similar needs can better maintain their custom platform capabilities.

[Randgalt]

This would be a bad direction for the project. There are better ways to allow custom clients than exposing the internals via inheritance.