WIP: Elm OAS3 CI tests

[eriktim]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
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Description of the PR

Add OAS3 integration tests for Elm. (Follow-up on #1104.)

• Escape special characters
• Support additionalProperties
• Support arrays of arrays
• Support duplicate enum values

[mxinden]

@trenneman what is the timeline for "Support additionalProperties"? This is a blocker for us in prometheus/alertmanager#1560 to use the openapi-generator project. Can I be of any help?

[eriktim]

Hi @mxinden
I had hoped to fix it in this PR, but getting this one done is a bit more complex than I anticipated. I will start working on a new PR for the additionalProperties first. Will come with an ETA once I've got a feeling on the work that needs to be done. See #1262.

[wing328]

@eriktim this PR has been opened for too long. Closing it for the time being. Please feel free to open a new one or reopen this one when it's ready for review.