Investigate if downleveled types are necessary

[chradek]

This package uses getters and setters in many of its exported classes (e.g. Connection). We know that starting with TypeScript 3.6, the generated types changed and are not compatible with versions of TypeScript older than 3.6. We should consider adding down-leveled types to the package to ensure TypeScript 3.1-3.5 users can continue compiling against this package.

/cc @ramya-rao-a