Upgrade to .NET 5.x

Upgrade to .NET 5.x
Added multi-framework target support - .NET 5, .NET Core 3.1, .NET Standard 2.1 and .NET Standard 2.0
Update libraries and SDKs to the latest version.

Upgrade to .NET Core 3.x

Upgrade all LiteX Sms packages to .NET Core 3.x
Upgrade all dependent Microsoft packages to .NET Core 3.x
Upgrade all dependent packages (SDKs) to the latest version

8.0.0 - Multiple Provider support

Multiple provider support (using provider factory)
Performance improvement in provider initialization stage.
Update libraries to latest version.

Codebreaking changes - Service method name changes: AddLiteXEmail changed to AddLiteXSmtpEmail

7.x.x - Before Multiple Provider support

Before multiple provider support (using provider factory)