Make methods that use settings non-static

[tomalec]

User developer story

As discussed in #398 (comment)

We use a lot of static functions that are not 100% "static," IMO.
PHP spec

Declaring class properties or methods as static makes them accessible without needing an instantiation of the class.

• public static function get_enabled_events uses self::get
• public static function load_opt_out uses self::get
• public static function get_product_identifier uses self::get
• public static function get uses the self::settings
  but we set the self::settings in the instance constructor https://github.com/woocommerce/woocommerce-google-analytics-integration/blob/cdb60c08a9cb62f351f5d324472ad62b6eef7105/includes/class-wc-google-gtag-js.php#L47:L47

So we could change self::get( * ) to $this->get() or even $this->settings[ * ] (IMHO, get just blurs the picture here w/o much benefit).

Is your feature request related to a problem?

It's not a big problem. This is a singleton, so the line between class and instance is blurred anyway. However, the static could mislead developers by suggesting no instance needs to be created to use them, which is not true.

How to reproduce the problem

Use WC_Google_Gtag_JS::get without creating the instance first.

Describe the solution you'd like

1. Turn settings to be instance propery
2. Turn all methods that use settings directly or indirectly to be non-static
3. Remove get function, as currently it limits the data types to be used as settings to string which may be problematic in the future

Describe alternatives you've considered

Do only 1-2 above.

Technical

Figma link

Acceptance criteria

Unknowns

• There may be some limitations, logic, or convention put around settings and get in the WC Core class WC_Integration

Out of bounds/rabbit holes

Event tracking