Add Ssh credentials support or open creadentionals class

[mcdis]

add to Native methods: git_cred_ssh_key_new and wrap it or open GitCredentialHandler in Credentials class:

  public abstract class Credentials
  {
    protected internal abstract int GitCredentialHandler(out IntPtr cred);
  }

remove internal and any can p/invoke selfmade

[vatt849]

+1 @ethomson ssh support is a very important thing for a lot of people, so if you don't want/can't do it by yourself, make possibility to implement it by myself, please

[umerenkovDaniil]