Including Distributed Postgres Cache information

[JaredMSFT]

We've recently released the Microsoft.Extensions.Caching.Postgres library and wanted to include this option in our docs.

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Internal previews

📄 File	🔗 Preview link
aspnetcore/fundamentals/app-state.md	Session and state management in ASP.NET Core
aspnetcore/performance/caching/distributed.md	Distributed caching in ASP.NET Core
aspnetcore/performance/caching/hybrid.md	HybridCache library in ASP.NET Core
aspnetcore/performance/caching/overview.md	Overview of caching in ASP.NET Core
aspnetcore/performance/caching/response.md	Response caching in ASP.NET Core

[JaredMSFT]

@dotnet-policy-service agree company="Microsoft"

[tdykstra]

This looks great; I just noticed a couple mentions of passwords and they need something said about secure handling of credentials. See the article I linked to in the comments below.

[wadepickett]

@tdykstra, adding myself as a backup reviewer per your request.

[wadepickett]

@JaredMSFT, this looks great!!. I only see a very minor item to consider:
Should "Postgres" should be "PostgreSQL" to avoid using two different names for the same thing? If so I can change that for you and publish, otherwise I will publish as is.