Add Currency Converter API connecting feature

[HirokazuNishi]

Description

This PR aims to add "Currency Converter API" connecting feature for retrieving TWD currency rate.
Currency rates services that Magento can connect by default can't retrieve TWD rates.

Fixed Issues (if relevant)

1. Cannot retrieve currency rates for TWD #15541: Cannot retrieve currency rates for TWD

Manual testing scenarios

1. Allow TWD from admin panel
2. Access to "Store > Currency Rates"
3. Choose "Currency Converter API" from the drop down
4. Click "Import" and wait
5. Currency rate for TWD will be set the form

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-cicd2]

All committers have signed the CLA.

[magento-engcom-team]

Hi @HirokazuNishi. Thank you for your contribution.
We will aim to release these changes as part of 2.3.0.
Please check the release notes for final confirmation.