[elm] fix for invalid code being generated for enums

[mawis]

Elm code generator produces invalid elm code if there are enums in the OpenAPI specification. As described in #8343 the problem is the missing encoder/decoder for the enum field in the encoder/decoder of the structure. With this patch the encoders/decoders are added as required.

@eriktim

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[wing328]

I've not tested it locally. It looks to be a good fix so I'll include this in the upcoming v5.3.0 release. Thanks for the PR.