[Feature Request] Microsoft.Identity.Abstractions should bring a net462 target framework

[jmprieur]

Is your feature request related to a problem? Please describe.
When net462 libraries (think Id.Web, SAL) take a dependency on netstandard2.0 libraries, this ends-up pulling the whole framework, whereas Microsoft.Identity.Abstractions is tiny and does not reference much. By adding direct support for net462, this enables to draw less dependencies

Describe the solution you'd like
Add the net462 target framework

Describe alternatives you've considered
None

Additional context
See also the common dependency management.

[jennyf19]

in 1.0.6-preview