Add return types to mturk function calls

[JamesPHoughton]

Currently, the functions defined in mturk.ts have parameter types, but don't define return types. Putting these in place will help us think through our API definition and ensure that we are using functions properly as we go forward.

Lets start by typing the following functions and get tests set up for them:

• [x] createHIT
• [x] listHITs

Resources:

• https://www.educba.com/typescript-function-return-type/