Support resource subscriptions

[chemicL]

This change allows clients to register subscriptions for resource updates.

This PR aims to supersede #838 and resolve #837 and #776.

Motivation and Context

This functionality was missing.

How Has This Been Tested?

Unit and integration tests added.

New API

Nothing breaking but new API is introduced:

• New default method McpServerTransportProviderBase#notifyClient(String sessionId, String method, Object params)
• New constructor in McpServerSession that accepts a Supplier<Mono<Void>> onClose callback
• New constructor in DefaultMcpStreamableServerSessionFactory that accepts a Function<String, Mono<Void>> onClose

Behaviour changes

• Previously, McpAsyncServer#notifyResourceUpdated broadcast to all connected clients. Now it only notifies sessions that have subscribed to the specific resource URI.
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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