Move currencies building process to a cronjob

[supercid]

Description

Removes the currency building from the settings observer, which run at every adminhtml login.

Related Issue

Not public

Motivation and Context

Merchants with a massive amount storeviews and currencies have trouble logging into admin if cache is cleared since we need to build the settings with all currencies for all storeviews. This offsets this operation to a cronjob, since we don't need to update currencies so often.

How Has This Been Tested?

Tested locally with:

bin/magento cron:install
bin/magento cron:run

Check on the database with:

SELECT job_code 
FROM cron_schedule 
WHERE job_code LIKE '%nosto%';
GROUP BY job_code

Documentation:

Nosto/wiki-docs#71

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.
• All new and existing tests passed.
• I have assigned the correct milestone or created one if non-existent.
• I have correctly labeled this pull request.
• I have linked the corresponding issue in this description.
• I have updated the corresponding Jira ticket.
• I have requested a review from at least 2 reviewers
• I have checked the base branch of this pull request
• I have checked my code for any possible security vulnerabilities