Include Connectors.AI.OpenAI as dependency of the SK main nuget

[shawncal]

Motivation and Context

When users install the "Microsoft.SemanticKernel" nuget package, it should have a fully working path without requiring them to seek additional nugets explicitly.

Description

Adds the SemanticKernel.Connectors.AI.OpenAI nuget package as a dependency of the main nuget package, so that users installing the main package will get this too.

Note: <PrivateAssets>none</> tells the hosting package to include all assets of the OpenAI package. The default value of PrivateAssets is "contentfiles;analyzers;build", so content files are considered "private" and hidden from the host project.