Multiple signature default configuration

[louisboulagnon]

Describe the bug
When several signature zones are defined in the config, only the first one is extracted in the DocusignBundle\DependencyInjection\Configuration.php

To Reproduce
Install and configuret the bundle, setting 2 signature zones

Expected behavior
See all configured signatures in the processed configuration

Screenshots

Screenshot of an xdebug breakpoint in DocusignExtension.php line 63, right after $config = $this->processConfiguration($configuration, $configs)

Additional context
I am by no means a configuration expert, so I am not sure how and what the change could impact and that's why I prefer posting an issue rather than a pull request directly, but I'm guessing there is something to fix in the Configuration.php file, around the line 121:123.
A fix I found was simply changing the line 123 from:
->then(static function ($v) { return ['default' => [$v[0]]]; }) to ->then(static function ($v) { return ['default' => $v]; })

[louisboulagnon]

I also discovered that configuring the docusign.yaml differently works without changing the code

    signatures:
        # this is an array of positions, you can have multiple signatures locations per pages
        default:
            -
                page: 1 # Default
                x_position: 200 # Top left corner in pixels
                y_position: 400 # Top left corner in pixels
            -
                page: 2 # Default
                x_position: 200 # Top left corner in pixels
                y_position: 400 # Top left corner in pixels