[5.x] Add entry password protection

[aerni]

This PR extends the existing password protection scheme with the concept of local passwords. This allows users to protect individual entries based on a password field.

To use a local entry password, you start by defining the field containing the password. In this example, we'll call the field password:

'password' => [
    'driver' => 'password',
    'allowed' => ['master-password'],
    'field' => 'password',
],

Next, you'll need to apply the protection scheme on the entry, along with the password.

id: 277de5b5-d28a-4af9-a57b-a929e39d59c9
title: 'Protected Page'
protect: password
password: local-password

Now, users can access the protected page using local-password.

If the protection scheme has both allowed and field configured, the passwords in allowed will act as master passwords. In our example, you'll be able to access the protected entry either with master-password or local-password.

If you use the local-password, the session will be scoped to the entry's ID. This ensures that you can only access the entry you entered the password for. This also ensures that multiple entries can share the same local password but still control access individually.

If you use the master-password, the session will be scoped to the protection scheme and immediately grant access to all other entries as well.

And if, for whatever reason, an entry uses a local password that is the same as a master password, access will be scoped to the entry as well. This ensures that global access won't be granted accidentally.

A little hack you can use to enable password protection only if an entry's password field is filled:

Collection::computed('pages', 'protect', function ($entry, $value) {
    if ($value) {
        return $value;
    }

    $field = config('statamic.protect.schemes.password.field');

    return $entry->$field ? 'password' : null;
});