Add x-go-custom-tag to go-server generator

[andrewkav]

PR checklist

• [x ] Read the contribution guidelines.
• [ x] 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\.
• [x ] Filed the PR against the correct branch: master (3.3.x), 4.0.x. Default: master.
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Description of the PR

The PR adds possibility to include custom tags into generated models
as described in #1145

@antihax @bvwells @grokify

[wing328]

@andrewkav as discussed, you may consider using the following to make the line a little bit shorter:

{{#vendorExtensions.x-go-custom-tag}} {{{.}}}{{/vendorExtensions.x-go-custom-tag}}

[andrewkav]

@wing328 I've updated the PR. Also added the same for go and go-gin generators to make it consistent.

[wing328]

@andrewkav thanks for the enhancement, which has been merged into master.

cc @antihax (2017/11) @bvwells (2017/12) @grokify (2018/07) @kemokemo (2018/09)

[wing328]

@andrewkav thanks again for the enhancement, which has been included in the v3.3.1 release: https://twitter.com/oas_generator/status/1052020299821080577

[zh4ngx]

Question about this - how would one reference the baseName in the yaml definition itself? For purposes of using the property name, or say a camelized version, as part of the custom tag?