WIP Add metadata propagation

[bogdang989]

Follow-up for common-workflow-language/common-workflow-language#710

[tetron]

This is cool. I owe you a code review.

[codecov]

Codecov Report

Merging #876 into master will increase coverage by 0.98%.
The diff coverage is 10.34%.

@@ Coverage Diff @@ ## master #876 +/- ## ========================================= + Coverage 72.71% 73.7% +0.98%  ========================================= Files 29 33 +4 Lines 5810 6462 +652 Branches 1454 1637 +183 ========================================= + Hits 4225 4763 +538  - Misses 1133 1244 +111  - Partials 452 455 +3

Impacted Files	Coverage Δ
cwltool/builder.py	80% <100%> (+16.36%)	⬆️
cwltool/command_line_tool.py	83.65% <7.14%> (+4.18%)	⬆️
cwltool/docker_id.py	47.91% <0%> (-2.09%)	⬇️
cwltool/context.py	96.46% <0%> (-0.87%)	⬇️
cwltool/flatten.py	78.94% <0%> (ø)
cwltool/__init__.py	100% <0%> (ø)
cwltool/main.py	32.5% <0%> (ø)
cwltool/utils.py	53.44% <0%> (ø)
cwltool/argparser.py	83.88% <0%> (+0.47%)	⬆️
cwltool/executors.py	76.79% <0%> (+2.2%)	⬆️
... and 13 more

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