refactor: move i/netx/archival structs to i/model (#659)

We recently started moving core data structures inside of the
internal/model package as detailed in https://github.com/ooni/probe/issues/1885.

The chief reason to do that is to have a set of fundamental
shared data types to help us rationalize the codebase.

This specific diff moves internal/netx/archival's core data types
inside the internal/model package. While there, it also refactors the
existing tests to improve their quality. Additionally, we also added
an extra test to ensure `ArchivalHTTPBody` is an alias for
`ArchivalMaybeBinaryData`, which is required to ensure the
custom JSON serialization process works for it.

We're doing that because both internal/netx/archival and
internal/measurex define their own archival data structures.

We developed measurex using its own structures because it
allowed to iterate more quickly. Now that we have sketched
out measurex, the time has come to consolidate.

My overall aim is to spend a few more hours this week on
engineering measurex. This work is preliminary work before
we finish up both measurex and websteps.

We described this cleanup in https://github.com/ooni/probe/issues/1957.

internal/measurex/resolver.go

@@ -26,7 +26,7 @@ func WrapResolver(begin time.Time, db WritableDB, r model.Resolver) model.Resolv
 }
 
 // NewResolverSystem creates a system resolver and then wraps
-// it using the WrapResolver function/
+// it using the WrapResolver function.
 func (mx *Measurer) NewResolverSystem(db WritableDB, logger model.Logger) model.Resolver {
 	return mx.WrapResolver(db, netxlite.NewResolverStdlib(logger))
 }