Update subscriptions schema to include manual discounts

[rubenharutyunov]

Description of change

Currently, tap's subscription schema doesn't include manual discounts. This PR updates the subscription schema to include missing discount attributes based on Chargebee API.

Manual QA steps

• Installed locally (python setup.py install), configured to use our Chargebee site
• Generated catalog (tap-chargebee --config config.json --discover > catalog.json), verified that it contains new attributes
• Ran the tap (tap-chargebee --config config.json --catalog catalog.json) and verified that RECORD documents now contain manual discounts (where applied)

Risks

None I can think of

Rollback steps

• revert this branch

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[singer-bot]

You did it @rubenharutyunov!

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[shantanu73]

Verified that this is a non-breaking change. Approving this.