Improve agent plugin nodeattestore x509pop's configuration handling

[edwbuck]

The x509pop agent plugin has a few key configuration items, but unlike a number of the other plugins, these configuration items seem to be validated in an intertwined way, with some of the validation logic being embedded in loadConfigData(...) which should be responsible for loading the data, not loading the data and also raising errors about the config.

This issue is to track the work specific to x509pop, which was deemed necessary (an a bit out of scope) during the work of #5303 which covers a broader effort of establishing a Validation interface in the plugins.