Allow to skip the CA certificate Validation. This fixes #6

[joaomatossilva]

No description provided.

[rolftimmermans]

I'm not sure about this. Shouldn't it be a compile time flag? Very few API users will create a Client directly.

In any case omitting ServerCertificateCustomValidationCallback does not mean that the certificate is not validated, it means that it's validated with the default roots, not the roots supplied in src/Tinify/data/cacert.pem.

[joaomatossilva]

In order to make it a compile time flag, you would need to create two diferent libraries. One with and another one without. I got the impression this was going just to be a fallback while Microsoft has issues with the just supporting the OpenSsl.

By ommitting the ServerCertificateCustomValidationCallback I was just falling back to the originalo behaviour of the HttpClient.
Do you mean we should skip validation at all?