Add total build time as metadata to artifacts

As it stands, buildstream doesn't contain the build time of an element in the resulting artifact. I believe including this would be useful information to keep track of.

The issue names no files, tests, or entry points. Start by locating artifact metadata handling and how build timing is currently measured or serialized; then trace the relevant tests. Done means the resulting artifact records total build time and the behavior is covered by tests.

Written by the indexing model from the issue text.