Add SandboxREAPI class

This adds the abstract SandboxREAPI class and moves a part of the functionality of SandboxRemote to the new class. This will allow code reuse between SandboxRemote and the planned buildbox-run sandboxing backend (#1177 (closed)).

It also fixes a small bug in SandboxRemote by ensuring the working directory exists in the input tree. This will be covered by the planned buildbox-run CI job.

I've extracted this from the WIP buildbox-run branch as this part should be ready for review.