Feature/daniele/checkout custom parameters

[DanieleAlessandra]

This PR introduces support for additional checkout configuration properties in the SDK initializer.

What’s new

The following properties are now recognized during initialization:

// currency
'currency'                      => 'string',
'default_currency'              => 'string',
// cart
'always_show_renewals_amount'   => 'bool',
'annual_discount'               => 'bool',
'billing_cycle'                 => ['string', 'int'],
'bundle_discount'               => 'float',
'maximize_discounts'            => 'bool',
'multisite_discount'            => ['bool', 'string'], // string expected to be "auto"
'show_inline_currency_selector' => 'bool',
'show_monthly'                  => 'bool',
// appearance
'form_position'                 => 'string',
'is_bundle_collapsed'           => 'bool',
'layout'                        => 'string',
'refund_policy_position'        => 'string',
'show_refund_badge'             => 'bool',
'show_reviews'                  => 'bool',
'show_upsells'                  => 'bool',
'title'                         => 'string',

Behavior
• If these properties exist in the initialization config, they are validated and stored.
• When opening the checkout (both iframe and redirect modes), the provided values are automatically passed along to the checkout context.

This allows developers to customize the checkout flow appearance and cart behavior more flexibly directly from the SDK initialization.

[fajardoleo]

@DanieleAlessandra An extra line.

[fajardoleo]

@DanieleAlessandra Missing spaces around $config.

[fajardoleo]

@DanieleAlessandra Please check the spaces.

[fajardoleo]

@DanieleAlessandra Spaces

[fajardoleo]

@DanieleAlessandra Curly braces

[fajardoleo]

@DanieleAlessandra Curly braces :)

[fajardoleo]

@DanieleAlessandra Please check this out:
https://developer.wordpress.org/coding-standards/wordpress-coding-standards/php/#declaring-arrays

Long array syntax.