PHP library is not PSR-4 compliant, autoloading doesn't work

The `composer.json` currently states this:

```php
"autoload": {
"psr-4": {
"FacebookAds\\": "php/capi-param-builder/src/"
}
},
```

Similarly, the `composer.json` within `php/capi-param-builder/php` states this:

```
"autoload": {
"psr-4": {
"FacebookAds\\": "src/"
}
```

The issue is with the classes within the subfolders of the PHP package (`model`, `piiUtil`, `util`) which all use `FacebookAds` namespace. The PSR-4 autoloading won't match them. Any attempt to use one of these classes ends up with straight PHP error, for example:

```
Class "FacebookAds\PII_DATA_TYPE" not found
```

Because of this, our conversion tracking is failing. The Business SDK library is using it exactly this way and fails: https://github.com/facebook/facebook-php-business-sdk/blob/main/src/FacebookAds/Object/ServerSide/UserData.php#L936

Please verify the correctness of autoloading and either update `composer.json` files, or move files out of their subfolders so they match PSR-4 standard.

Thanks!

Inspect the root composer.json and php/capi-param-builder/php/composer.json, then compare their PSR-4 mappings with the namespaces and paths under php/capi-param-builder/src/, especially model, piiUtil, and util. Regenerate Composer autoloading and verify that classes such as FacebookAds\PII_DATA_TYPE can be loaded by the Business SDK usage described in the issue.

Written by the indexing model from the issue text.