Include what you use: add direct includes in files

[wendythewan]

Problem

What is being fixed? Examples:

• Add direct includes for things used in the file.

Change overview

• Add direct includes for things used in the file.
• It isn't necessary to compile, but it's necessary to satisfy IWYU checkers (which includes our internal tools). They can help SDK consumers satisfy their own tooling needs.

Testing

How was this tested? (at least one bullet point required)

• Compiled.