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/engine/probeservices/checkin.go

@@ -3,18 +3,18 @@ package probeservices
 import (
 	"context"
 
-	"github.com/ooni/probe-cli/v3/internal/engine/model"
+	"github.com/ooni/probe-cli/v3/internal/model"
 )
 
 type checkInResult struct {
-	Tests model.CheckInInfo `json:"tests"`
-	V     int               `json:"v"`
+	Tests model.OOAPICheckInInfo `json:"tests"`
+	V     int                    `json:"v"`
 }
 
 // CheckIn function is called by probes asking if there are tests to be run
 // The config argument contains the mandatory settings.
 // Returns the list of tests to run and the URLs, on success, or an explanatory error, in case of failure.
-func (c Client) CheckIn(ctx context.Context, config model.CheckInConfig) (*model.CheckInInfo, error) {
+func (c Client) CheckIn(ctx context.Context, config model.OOAPICheckInConfig) (*model.OOAPICheckInInfo, error) {
 	var response checkInResult
 	if err := c.Client.PostJSON(ctx, "/api/v1/check-in", config, &response); err != nil {
 		return nil, err