Lower required net framework

[damtur]

Description

The goal for this PR is to make it easier to consume the Mjolnir library on more platforms. It will be possible to use it by everything that targets netstandard1.2 or greater.
We've also made properties of the MjolnirConfiguration class virtual and the class is no longer sealed. This will allow easier adaptation of the configuration of Mjolnir.
From this version we also include a <FileVersion> and <AssemblyVersion> locked on v3.0.0.0. This is for easier dependency management with consuming applications.

All changes here are backwards compatible.

Changes

Enhancements

• Add virtual modifiers to MjolnirConfiguration class and remove the sealed keyword from the class.
• Lower target framework to netstandard1.2
• Add a new protected constructor to AsyncCommand and SyncCommand to allow for extensions of the classes to override the command name.

Bug fixes

• Fix bug with not being able to serialize exception due to GroupKey not being serializable

Other

• Remove global.json content (specific SDK requirement)
• Include File and assembly version in csproj.

[lewislabs]

👍