build-unit-test-docker: Add span-lite
build-unit-test-docker: Add span-lite

Spans are nice to have as they are a c++20 approved, generalized version
of a mutable view of arbitrary data types. This implementation is
minimal and mostly conforms to the standard, with a clear migration path
when we have c++20 available. This will be first used by stdplus.

Change-Id: I8219f1cf452919044970841d7561c50ebc8e81a0
Signed-off-by: William A. Kennington III <30fff5198f4d10ca1117e5f3ec82f3b4b8900d81@google.com>

openbmc/openbmc-build-scripts,openbmc/openbmc-build-scripts