Port D2L.Security.OAuth2 to .NET core

[x] Remove System.Web.HttpMessage overloads
[x] Switch from other serialization stuff to Newtonsoft.Json
[x] Switch from System.Runtime.Caching to Microsoft.Framework.Caching.Memory for MemoryCache (possible breaking API changes?)
[x] The type or namespace name AsymmetricSignature(De)formatter could not be found [OS: #36]
[x] Possible SecurityKey/SecurityToken breaking API changes [OS: #36]
[x] Where are JwtSecurityToken and JwtSecurityTokenHandler
[x] D2LSecurityToken does not implement inherited abstract member SecurityToken.SigningKey.get [OS: #36]
[x] Bring in DNX version of D2L.Services.Core.Extensions.
[x] Build/bring in DNX version of D2L.Services.Core.Exceptions
[ ] Figure out Newtonsoft.Json
[x] Netstandard D2L.CodeStyle.Annotations (https://github.com/Brightspace/D2L.CodeStyle/pull/457) [OS: https://github.com/Brightspace/D2L.Security.OAuth2/pull/109]

Brightspace / D2L.Security.OAuth2