[HttpFoundation][2.1] Allow MimeTypeGuesser to be constructed and customized outside of the singleton scope

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #5615
| License       | MIT

Allow singleton customization with the method `defineAsInstance`.

For example, to use a unique `CustomMimeTypeGuesser` : 

```php
$guesser = new MimeTypeGuesser();
$guesser->register(new CustomMimeTypeGuesser());
$guesser->defineAsInstance();
```

the following call to `MimeTypeGuesser::getInstance();` will return the previously defined `$guesser`.