ServerOptions: Add WithInstructions

[njern]

Sweep Summary

Adds a new WithInstructions server option to allow users to set instructions describing how to use the server and its features.

• Added WithInstructions function in server.go that creates a ServerOptions function to set the server's instructions.
• The instructions will be available to clients through the initialize response, helping users understand how to use the server.

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Simple change, let users set the instructions describing how to use the server and its features

[Copilot]

Pull Request Overview

This PR adds a new server option "WithInstructions" to allow users to set the server's usage instructions via a functional option.

• Added WithInstructions function that accepts a string and sets it on the Server struct.
• Returns a closure that assigns the instructions pointer to the Server instance.

[ecekyn]

duplicate of #102, which has just been merged.