[Bug] Cannot Access HostedFieldsInstance.on() method without removing type

[stefanmterry]

🐞 Describe the Bug

The return type from HostedFields.render() does not include the on() function to access field event subscription. It must by type-casted as "any" to avoid typescript errors.

🔬 Minimal Reproduction

In the .then callback from a HostedFields.render() function, try to access the .on() function:

loadScript({
...
}).then(
  (paypal) => {
    paypal.HostedFields.render({
      ...
    }).then(
      (cardFields) => {
          cardFields.on('validityChange', (event) => { ... });
      }
    )
  }
)

throws an "method on() does not exist on type HostedFieldsHandler" error

...
.then(
  (cardFields) => {
    (cardFields as any).on('validityChange', (event) => { ... });
  }
)

subscribes to the event just fine.

😕 Actual Behavior

The interface is missing declarations for the .on() subscription method.

🤔 Expected Behavior

The interface includes all method calls available through the Paypal API

🌍 Environment

• Node.js/npm: - 6.14.15
• OS: - Ubuntu 20.04.5
• Browser: - Chrome

➕ Additional Context

While not a critical breaking issues, as it's easily worked around, it was definitely a source of confusion.