Update Elm for elm/http 2.0.0

Created by: 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\.
• Filed the PR against the correct branch: master, 3.4.x, 4.0.x. Default: master.
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Description of the PR

Upgrade the generator to use the new elm/http package. This changes the way all APIs calls are called.

addPet : Pet -> Http.Request ()

becomes

addPet :
    { onSend : Result Http.Error () -> msg
    , body : Pet
    }
    -> Cmd msg

opening the way for additional flags to allow adding custom baseUrls, timeouts, trackers and headers (in follow-up PRs). This PR already adds support for query parameters and escapes all parameters safely by using elm/url.

cc @wing328

Fixes #1461 (closed).