allow providing a custom certificate store

[bdegomme]

I recently has an issue with a customer having a valid certificate chain, but nevertheless the viewpoint gem raised error certificate verify failed (unable to get local issuer certificate) (OpenSSL::SSL::SSLError) when trying to access their server. Their certificate was generated with Let's Encrypt.

I fixed this error by providing the default certificate store for use by EWSClient, like this

cert_store = OpenSSL::X509::Store.new
cert_store.set_default_paths
Viewpoint::EWSClient.new(url, username, password, http_opts: { cert_store: })

I'm not sure why Viewpoint doesn't use the system certificate store by default, and there may be a more elegant fix to this, but I thought I would put this out there.