Allow JsonSerializerOptions parameters for user-defined and inputs and hardcode everything else to the source generator.

[eiriktsarpalis]

This PR updates the JSON serialization aspects of the IMcpClient and IMcpServer abstractions in the following ways:

• Removes serialization concerns from the core IMcpEndpoint signature by making its SendRequestAsync non generic.
• JSON-RPC parameter and result serialization is now exclusively handled using extension methods over the abstractions.
• Extension methods accepting user-defined objects now accept an optional JsonSerializerOptions overload that can be used to override serialization settings.
• Serialization of the built-in implementations now hardcode to the source generator where applicable.

[jeffhandley]

Adding the breaking-change label retroactively during release notes revision.