smith: add obfuscation functionality

[lrlna]

When starting apollo-smith using with_document, we want to be able to mangle a lot of the idents in a schema, specifically a supergraph, replacing them with an arbitrary, valid ident.

Some items should remain the same in order for this to be a valid supergraph, such as any federation directives and their arguments, as well as the object types and enums that are used in those arguments.