Update .NET example to include non-admin logic and separated auth

[danielfrankcom]

This PR updates the .NET C# example.

It includes the following changes:

• Synchronize the example with the publicly available docs here
• Synchronize the example with other approved examples
  • Generate password through SDK rather than custom request
  • Remove UUID generation
  • Align create table DDL
• Clean up and improve the documentation/comments
• Minimize dependencies and use non-preview versions
• Target netstandard2.0 for broader compatibility
• Add non-admin connection to existing example
• Allow configuration with environment variables

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