Add user agent string in requests to Mollie

[remcotolsma]

For partner commission we may need to add a user agent string to the requests to Mollie.

https://docs.mollie.com/integration-partners/user-agent-strings

Should probably be something like this:

PronamicPay/9.0.0 WordPress/6.0.2

https://developer.wordpress.org/reference/classes/WP_Http/request/

Default 'WordPress/' . get_bloginfo( 'version' ) . '; ' . get_bloginfo( 'url' ).

https://github.com/WordPress/WordPress/blob/f9db66d504fc72942515f6c0ed2b63aee7cef876/wp-includes/class-wp-http.php#L183-L192

cc @LeoOosterloo

[remcotolsma]

Added, user agent example:

PronamicPay/9.1.0 WordPress/6.0.2;https://www.pronamic.eu/