Use braintree component when using gateway

[salwator]

What I'm trying to achieve

We do want to take full advantage of Braintree library possibilities, like choosing an existing payment source when client token was generated for returning customer.

Describe a proposed solution

Use something like this.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.