refactor: interfaces and data types into the model package (#642)

## Checklist

- [x] I have read the [contribution guidelines](https://github.com/ooni/probe-cli/blob/master/CONTRIBUTING.md)
- [x] reference issue for this pull request: https://github.com/ooni/probe/issues/1885
- [x] related ooni/spec pull request: N/A

Location of the issue tracker: https://github.com/ooni/probe

## Description

This PR contains a set of changes to move important interfaces and data types into the `./internal/model` package.

The criteria for including an interface or data type in here is roughly that the type should be important and used by several packages. We are especially interested to move more interfaces here to increase modularity.

An additional side effect is that, by reading this package, one should be able to understand more quickly how different parts of the codebase interact with each other.

This is what I want to move in `internal/model`:

- [x] most important interfaces from `internal/netxlite`
- [x] everything that was previously part of `internal/engine/model`
- [x] mocks from `internal/netxlite/mocks` should also be moved in here as a subpackage

internal/netxlite/dnsoverhttps.go

@@ -8,12 +8,13 @@ import (
 	"time"
 
 	"github.com/ooni/probe-cli/v3/internal/engine/httpheader"
+	"github.com/ooni/probe-cli/v3/internal/model"
 )
 
 // DNSOverHTTPS is a DNS-over-HTTPS DNSTransport.
 type DNSOverHTTPS struct {
 	// Client is the MANDATORY http client to use.
-	Client HTTPClient
+	Client model.HTTPClient
 
 	// URL is the MANDATORY URL of the DNS-over-HTTPS server.
 	URL string
@@ -30,14 +31,14 @@ type DNSOverHTTPS struct {
 // - client in http.Client-like type (e.g., http.DefaultClient);
 //
 // - URL is the DoH resolver URL (e.g., https://1.1.1.1/dns-query).
-func NewDNSOverHTTPS(client HTTPClient, URL string) *DNSOverHTTPS {
+func NewDNSOverHTTPS(client model.HTTPClient, URL string) *DNSOverHTTPS {
 	return NewDNSOverHTTPSWithHostOverride(client, URL, "")
 }
 
 // NewDNSOverHTTPSWithHostOverride creates a new DNSOverHTTPS
 // with the given Host header override.
 func NewDNSOverHTTPSWithHostOverride(
-	client HTTPClient, URL, hostOverride string) *DNSOverHTTPS {
+	client model.HTTPClient, URL, hostOverride string) *DNSOverHTTPS {
 	return &DNSOverHTTPS{Client: client, URL: URL, HostOverride: hostOverride}
 }
 
@@ -89,4 +90,4 @@ func (t *DNSOverHTTPS) CloseIdleConnections() {
 	t.Client.CloseIdleConnections()
 }
 
-var _ DNSTransport = &DNSOverHTTPS{}
+var _ model.DNSTransport = &DNSOverHTTPS{}