Let Container and Scope classes implement IAsyncDisposable

[dotnetjunkie]

Feature #778 adds async disposable support for ASP.NET Core, but even outside the realm of ASP.NET Core, async disposable support is useful in two places:

• To asynchronously dispose of disposables on a Scope
• To asynchronously dispose the Container.

Because of the extra required Microsoft.Bcl.AsyncInterfaces package, I feel adding support to the core library is unsuited for v4, but we might want to reconsider for v5.

[dotnetjunkie]

feature-791 branch created.

[dotnetjunkie]

The Microsoft.Bcl.AsyncInterfaces library requires >= .NET 4.6.1 or >= .NET Standard 2.0. The IAsyncDisposable interface is included in .NET Standard 2.1. Because of this constraint, 2 new build outputs will be added for the Simple Injector core library, namely:

• .NET 4.6.1
• .NET Standard 2.1

This means that the following following Simple Injector builds will not support IAsyncDisposable:

• .NET Standard 1.0
• .NET Standard 1.3
• .NET 4.5