Jmprieur/add target462

[jmprieur]

Fixes #48 and #45

• Adds netstandard2.1 (because HttpMethod.Patch is widely used).
• Adds the Patch methods to IDownstreamRestApi
• and net462 (to avoid drawing the entiere framework when this
  library is consumed from a library targeting 462)

Dependencies:

dotnet list src\Microsoft.Identity.Abstractions\Microsoft.Identity.Abstractions.csproj package --include-transitive
Project 'Microsoft.Identity.Abstractions' has the following package references
   [net462]:
   Top-level Package                  Requested   Resolved
   > Microsoft.SourceLink.GitHub      1.0.0       1.0.0
   > System.Net.Http                  4.3.4       4.3.4

   Transitive Package                                   Resolved
   > Microsoft.Build.Tasks.Git                          1.0.0
   > Microsoft.SourceLink.Common                        1.0.0
   > System.Security.Cryptography.Algorithms            4.3.0
   > System.Security.Cryptography.Encoding              4.3.0
   > System.Security.Cryptography.Primitives            4.3.0
   > System.Security.Cryptography.X509Certificates      4.3.0

   [netstandard2.0]:
   Top-level Package                     Requested   Resolved
   > Microsoft.SourceLink.GitHub         1.0.0       1.0.0
   > NETStandard.Library           (A)   [2.0.3, )   2.0.3

   Transitive Package                 Resolved
   > Microsoft.Build.Tasks.Git        1.0.0
   > Microsoft.NETCore.Platforms      1.1.0
   > Microsoft.SourceLink.Common      1.0.0

   [netstandard2.1]:
   Top-level Package                  Requested   Resolved
   > Microsoft.SourceLink.GitHub      1.0.0       1.0.0

   Transitive Package                 Resolved
   > Microsoft.Build.Tasks.Git        1.0.0
   > Microsoft.SourceLink.Common      1.0.0

[jennyf19]