[elm] Add support for additionalProperties (map/hash/dictionary)

[eriktim]

Fixes #1140.

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Description of the PR

Add support for additionalProperties

• Elm 0.18 is not supported, as there is no standard Dict encoder;
• @wing328 do all generators process additionalProperties in the parent alias? When I run the example from [elm] failed to generate map from object with additionalProperties #1140, alert already contains the map and labelSet becomes obsolete.

[eriktim]

@wing328 updated as discussed

[wing328]

@trenneman thanks. The change looks good to me.

As discussed the top level "additionalProperties" should be treated as an alias instead of a separate model. We'll address that in a separate PR.

[andys8]

Tested it, looking good :)

Is there documentation somewhere to know that --additional-properties elmPrefixCustomTypeVariants=true exists?

[eriktim]

Have a look at https://github.com/OpenAPITools/openapi-generator/blob/master/docs/generators/elm.md or the other generators in that directory.

[andys8]

Thanks :)