Add setting to store future subscriptions

What does this MR do and why?

Add future_subscriptions column in ApplicationSetting, and store received value during seat link.

This is for self managed EE to store an array of JSON objects during seat link.

ApplicationSetting is used instead of a new table because this will be overwritten during each sync, and won't be updated.

There will ever only be a handful of items, and are displayed as a whole, therefore no index is required.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Database Migration

Up

ALTER TABLE "application_settings" ADD "future_subscriptions" jsonb DEFAULT '[]' NOT NULL

Down

ALTER TABLE "application_settings" DROP COLUMN "future_subscriptions"

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #338657 (closed)