* Use BouncyCastle ECDsa when runtime is Mono
* Falls back to use BouncyCastle if CngKey.Import throws NotImplementedException (in Mono)
* Take NETStandard into consideration
* Adjust some comments
* Change #if NETFRAEWORK to #if NET462 for CngKey
* Separate implementations
* Consolidate Ecdsa property and HashAlgorithm property
* Rename Import_Cng and Import_Bcl to Import; Rename Export_Cng and Export_Bcl to Export;
* Add comments
* refactor
* add host key tests
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Co-authored-by: Robert Hague <rh@johnstreetcapital.com>
Co-authored-by: Rob Hague <rob.hague00@gmail.com>
