Make config::Config compatible with xDS manager

[XAMPPRocky]

Currently the xDS management server uses a slightly different file configuration than the quilkin client, because the Quilkin client requires some fields that aren't needed for the xDS manager. I think to simplify the code, that we re-use the same configuration type for both sides. To make this work we would just need to make a few changes to the schema to make it compatible with both agents. Mainly removing the dynamic/static distinction, and allowing all the current sub-properties to be optional in the root object. This would allow both client and server to read the same properties.