Remove DropInRequest Parameter From Constructor

[sshropshire]

DropInClient#launchDropIn(dropInRequest)

This PR moves all DropInClient internal logic into an internal class. Now merchants can launch DropIn without needing to know the exact composition of a DropInRequest when creating an instance.

Summary of changes

• Deprecate constructors that require a DropInRequest.
• Add DropInClient#launchDropIn(DropInRequest) method.
• This should help prevent issues like #355 from happening.

Checklist

• [x] Added a changelog entry

Authors

List GitHub usernames for everyone who contributed to this pull request.

• @sshropshire