AgentClient - preparation for changes in the initialization process

[tomaszgolebiowski]

This commit refactors the solution and prepares foundation for proper initialization process:

• AgentConnector is now renamed to AgentClient
• IAgentClient is now renamed to IAgentService
• It's now possible to have more than one class with notification handlers (e.g., one class for handling webview notifications and another class for handling debug/message notifications)
• There's no longer a need to use a factory to create the agent client
• Warnings are now made useful again
• Eliminated the usage of libraries containing known vulnerabilities
• Removed unnecessary references from Cody.Core
• Replaced packages.config with PackageReference