Support ECDSA signing keys

[kevinchalet]

IdentityModel supports ECDSA keys, but ASOS' discovery endpoint doesn't expose them yet. This is something I'd really like to fix in the next version.

Unfortunately, supporting EDCSA will have 2 side effects:

• Referencing the ECDSA types will likely crash on Mono, where these types don't exist.
• ECDsa.ExportParameters is only available on .NET Standard 1.6/.NET Desktop 4.6.3 (ASOS currently supports .NET Desktop 4.5.1 and .NET Standard 1.4).

Question: do we still need/want to support <.NET Desktop 4.6.3 and <.NET Standard 1.6?

[kevinchalet]

ASOS will continue to support net451 and netstandard1.4 but ECDSA keys will only be supported when targeting netstandard1.6 (and the future net463 TFM).

Note: the OWIN/Katana version won't have ECDSA support as there's no ECDsaSecurityKey in IdentityModel 4.5.