[C++] Draft initial public APIs for Datasets project #21960

The objective of this is to ensure general alignment with the discussion document

https://docs.google.com/document/d/1bVhzifD38qDypnSjtf8exvpP3sSB5x_Kw9m-n66FB2c/edit?usp=sharing

so that an initial working implementation can begin to take place

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

PRs and other links:

• GitHub Pull Request #4483

_{Note: This issue was originally created as ARROW-5512. Please see the migration documentation for further details.}