[V6] Accept email in `BTPayPalVaultRequest`

[scannillo]

Summary of changes

• This code change was already reviewed and approved in https://github.com/braintree/braintree_ios/pull/1188
• This PR instead moves the feature to the main branch to be released outside of the ShopperInsights beta feature release.

Checklist

• [X] Added a changelog entry

Authors

@scannillo

[scannillo]

Waiting to merge & release until feature works in production. Waiting for internal confirmation.