Add fourth MC Setup step value: 'store_requirements'

[layoutd]

Changes proposed in this Pull Request:

Goes with #927 .

This adds a fourth step value to the possible replies from GET /wc/gla/mc/step: store_requirements.

This value is returned when shipping rate, shipping time, tax rate and free shipping threshold values are all deemed to have been completed:

• Tax rate: US isn't a target country, or the tax_rate setting isn't empty
• Offers Free Shipping: not offered (offers_free_shipping is false), or offered and free_shipping_threshold set (can be 0, just not null)
• Shipping Rate: shipping_rate setting set to manual OR one row in wp_gla_shipping_rates table for each target country
• Shipping time: same as shipping rate

Detailed test instructions:

1. Go through MC Onboarding steps
2. After each step, make a GET /gla/mc/setup request and confirm that the step value is as expected
3. For shipping at settings, try different configurations to confirm that the values are correct:
   • If Shipping Rate and Time set to "Set in MC": store_requirements
   • If Shipping Rate and Time set to manual, but no values added: shipping_and_taxes
   • If Shipping Rate and Time set to manual, with values: store_requirements
   • Store has US as target country, but no option selected for taxes: shipping_and_taxes
   • Store has US as target country, EITHER option selected for taxes: store_requirements
   • Store has free shipping for some orders selected, empty value for the threshold: shipping_and_taxes
   • Store has free shipping for some orders selected, any value for the threshold (including 0 🤷🏻 ): store_requirements

(Changelog added with frontend changes).

Changelog entry

[jconroy]

@eason9487 can you check if this works for you?

[eason9487]

Apart from the PHP code that I can't evaluate, I tested this PR yesterday by changing the settings in step 3, and all logic works correctly.

[nima-karimi]

Looks good overall, added two comments.