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/ooapi/dependencies.go

@@ -4,6 +4,8 @@ import (
 	"context"
 	"io"
 	"net/http"
+
+	"github.com/ooni/probe-cli/v3/internal/model"
 )
 
 // JSONCodec is a JSON encoder and decoder. Generally, we use a
@@ -57,10 +59,4 @@ type GobCodec interface {
 // KVStore is a key-value store. This is the interface the
 // client expect for the key-value store used to save persistent
 // state (typically on the file system).
-type KVStore interface {
-	// Get gets a value from the key-value store.
-	Get(key string) ([]byte, error)
-
-	// Set stores a value into the key-value store.
-	Set(key string, value []byte) error
-}
+type KVStore = model.KeyValueStore