Introduce new meta package to install core packages

[dluc]

Change Microsoft.SemanticKernel to be a metapackage that installs 2 packages:

• Microsoft.SemanticKernel.Core: core code (previously named Microsoft.SemanticKernel) + abstractions
• Microsoft.SemanticKernel.Connectors.AI.OpenAI: OpenAI clients, depending on Azure OpenAI SDK and Azure Core

Benefits:

• Users installing Microsoft.SemanticKernel have all the code like before to use OpenAI
• Users can install the core kernel without OpenAI if they need to, e.g. when working with other AI providers
• We can use the metapackage to include more useful dependencies if needed, without having to put all in the same package