Privacy Pro Subscriptions: Update some internal models and methods

[nalcalag]

Task/Issue URL: https://app.asana.com/0/1201807753394693/1209007672205184/f

Description

Update some subscriptions models in order to support free trials on mobile:

• SubscriptionOffer model
• OptionsJson model
• getSubscriptionOffer() method

Steps to test this PR

Pre steps

• Apply patch to being able to test subscriptions in staging

Privacy Pro Eligible US

• Make sure you are US eligible
• Install form branch
• Check browser works as expected
• Go to Settings
• Check you can see Privacy Pro option
• Tap on that option
• Check Subscription page looks as expected
• Purchase any of the options and check it works well
• Go to Subscription Settings
• Check everything works as expected

Privacy Pro Eligible ROW (Optional)

• Make sure you are ROW eligible
• Install form branch
• Check browser works as expected
• Go to Settings
• Check you can see Privacy Pro option
• Tap on that option
• Check Subscription page looks as expected
• Purchase any of the options and check it works well
• Go to Subscription Settings
• Check everything works as expected

Privacy Pro No Eligible

• Install form branch
• Check browser works as expected
• Go to Settings
• Check you can't see Privacy Pro option

No UI changes

[nalcalag]

• Privacy Pro Subscriptions: Update some internal models and methods #5402 👈 (View in Graphite)
• develop

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[lmac012]

LGTM