use proxy setting in Spotfire.Dxp.Worker.Host.exe.config to perform mod signature validation

Mods' signature is validated when Analyst/Consumer utilize it in a analysis.

The signature is considered valid if certificate chain is built successfully and revocation verification is successful and it may require internet connection.

For Web Player, the server may need to go through a proxy to access the internet, so we need to configure Web Player to acknowledge the proxy.

Curretly proxy setting in Spotfire.Dxp.Worker.Host.exe.config is not applied when validating mod. One way to workaround this is to create a service account and set proxy in Internet Options to this account.

Please make mod signature validation acknowledge the proxy setting in Spotfire.Dxp.Worker.Host.exe.config.