Add mill cert example

[OR13]

• Added example vc-json-schema document for steel mill cert test report
• Added example vc for steel mill cert test report which uses the schema
• Added test for signing the the schema with jsonld
• Added test for issuing/verifying the steel mill cert test report vc
• Added test for verifying the unsigned mill cert vc matches the vc-json-schema

[OR13]

Sorry this is one commit :(

I spent a lot of time sanity checking jsonld issues, and chasing documentLoader bugs... still 35 changes in 1 commit is pretty sad.

[decentralgabe]

I am not qualified to review javascript, but the rest looks good

[OR13]

I'm gonna gather a bit more feedback from team / community before merging this.

[gjgd]

One test is not passing on my machine. What version on node does this require?

[OR13]

@gjgd requires node 12

[wyc]

Tests pass on my end:

% node --version
v13.3.0

Results:

Test Suites: 3 passed, 3 total
Tests:       3 passed, 3 total
Snapshots:   0 total
Time:        5.113s
Ran all test suites.