[go-experimental] Support selection and formatting of endpoint servers

[jirikuncar]

Support selection and formatting of endpoint servers using context.

Addresses #590

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
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cc @antihax (2017/11) @bvwells (2017/12) @grokify (2018/07) @kemokemo (2018/09) @bkabrda (2019/07)

[jirikuncar]

Should I create the same patch also for go templates?

[jirikuncar]

⬆️ @antihax (2017/11) @bvwells (2017/12) @grokify (2018/07) @kemokemo (2018/09) @bkabrda (2019/07)

[zippolyte]

This looks good to me ! thanks @jirikuncar

[bkabrda]

This looks really great overall. I left couple minor comments to fix, mostly improving error messages. We will also need to document this properly so that users know how to use all this functionality from the autogenerated docs.

As for including this in the stable go templates, I don't think we should do that. It breaks API of the generated clients (e.g. it removes Configuration.BasePath), so that's a no-go.

[bkabrda]

LGTM now (assuming CI run passes :)). Thanks!

[jirikuncar]

All green 🎉 @wing328 🙏

[jimschubert]

Thanks! This looks awesome and I'm looking forward to seeing positive use cases in the wild.