api: *: add API support for macros

This patch adds support to the server and client APIs to support macros.
The URI /build now accepts another argument, 'macros' which is a mapping
of macros which will be passed to the build before executing it. In the
client, this can be accessed by stringing together -D<VAR>=<VALUE>
arguments.

Docker-DCO-1.1-Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)