RSAUtil.GetCertFromPEMFiles throws PlatformNotSupportedException on Windows

**Output of `dotnet --info`:**

.NET Core SDK (reflecting any global.json):
Version: 2.1.402
Commit: 3599f217f4

Runtime Environment:
OS Name: Windows
OS Version: 10.0.17134
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\2.1.402\

Host (useful for support):
Version: 2.1.4
Commit: 85255dde3e

.NET Core SDKs installed:
1.0.0-preview2-003121 [C:\Program Files\dotnet\sdk]
1.0.0-preview2-003131 [C:\Program Files\dotnet\sdk]
2.0.3 [C:\Program Files\dotnet\sdk]
2.1.2 [C:\Program Files\dotnet\sdk]
2.1.4 [C:\Program Files\dotnet\sdk]
2.1.100 [C:\Program Files\dotnet\sdk]
2.1.101 [C:\Program Files\dotnet\sdk]
2.1.104 [C:\Program Files\dotnet\sdk]
2.1.201 [C:\Program Files\dotnet\sdk]
2.1.202 [C:\Program Files\dotnet\sdk]
2.1.402 [C:\Program Files\dotnet\sdk]

.NET Core runtimes installed:
Microsoft.AspNetCore.All 2.1.4 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.1.4 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 1.0.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 1.0.1 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.0.3 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.0.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.0.6 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.0.7 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.0.9 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.1.4 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]

**What version of Docker.DotNet?:**

Docker.DotNet.X509 - 3.125.2

**Steps to reproduce the issue:**
1. Invoke RSAUtil.GetCertFromPEMFiles(string certFilePath, string keyFilePath) with a valid certificate and key file on Windows 10.

**What actually happened?:**
RSAUtil.GetCertFromPEMFiles throw a PlatformNotSupportedException exception

Exception: System.PlatformNotSupportedException: Operation is not supported on this platform.
at System.Security.Cryptography.X509Certificates.X509Certificate2.set_PrivateKey(AsymmetricAlgorithm value)
at Docker.DotNet.X509.RSAUtil.GetCertFromPEMFiles(String certFilePath, String keyFilePath)

**What did you expect to happen?:**

I expected RSAUtil.GetCertFromPEMFiles to return a valid X509Certificate2 that represented the client certificate.

**Additional information:**

None