Use API key auth in templates by default

Motivation and Context

Description

• Switch to API Key for auth method
• Merge the 2 main bicep files / factor our common code
• Allow passing of models to use in new deployments too

Contribution Checklist

• [ ] The code builds clean without any errors or warnings
• [ ] The PR follows SK Contribution Guidelines (https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
• [ ] The code follows the .NET coding conventions (https://learn.microsoft.com/dotnet/csharp/fundamentals/coding-style/coding-conventions) verified with dotnet format
• [ ] All unit tests pass, and I have added new tests where possible
• [ ] I didn't break anyone :smile: