fix(payment): add joi validation on POST body for payment onboarding

[KenLSM]

Problem

Our payment/onboarding url doesn't strictly validates that the email field exists. This in turn causes some parts of the email validator to fail.

Notes: this was from a tester trying to inject a malicious request into our DB query.

Solution

Require email to be a valid string.

Breaking Changes

• No - this PR is backwards compatible

Tests

Regression

• Visit form.gov.sg/payments
• Enter your whitelisted email
• Ensure that the message "An email has been sent to this email address." is displayed
• Ensure that the email is received in your inbox

[g-tejas]

LGTM!