feat: introduce Cargo features to isolate client and server code

[hashemix]

📌 Summary

This PR introduces Cargo feature flags to modularize the client and server components of the crate.
The goal is to improve code separation, reduce unnecessary compilation, and allow for more targeted builds (e.g., only client, only server, or both).

🔍 Related Issues

• Fixes separate out client vs server #12

✨ Changes Made

• Introduced two new features client and server.
• Documented the new client and server feature flags in the README
• Updated examples to explicitly set the features needed
• Removed unnecessary dependencies from examples

💡 Additional Notes

Using Only the server Feature (you may also want to include the macros feature)

[dependencies]
rust-mcp-sdk = { version = "0.2.0", default-features = false, features = ["server"] }

Using Only the client Feature

[dependencies]
rust-mcp-sdk = { version = "0.2.0", default-features = false, features = ["client"] }