[Golang] Allow retrieving the underlying configuration for APIClient

[arvindth]

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, ./bin/openapi3/{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\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
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Description of the PR

(details of the change, additional tests that have been done, reference to the issue for tracking, etc)

• Needed for dynamically changing the underlying implementations and for testing
• Fixes [Golang][client] Allow access to APIClient.cfg.HTTPClient & APIClient.cfg.BasePath #1321, Fixes [REQ][Golang][Client] Add *http.Client Accessor #3412
• FYI, I considered exporting the cfg variable instead, but thought this was a more backward compatible change.
• Travis build on my branch passed.

cc @antihax @bvwells @grokify @kemokemo @bkabrda since this is a Go related change.

[bkabrda]

LGTM 👍 could come handy in some situations.

[bkabrda]

Oh BTW could you do a similar change to the go-experimental templates, @arvindth.

[bkabrda]

@wing328 I think this PR was closed by mistake, IMO the #3996 PR that was merged was supposed to resolve a different issue.

[antihax]

We should add a warning that modifying the configuration while live can cause data races and potentially unwanted behavior. It was intentionally protected.

[arvindth]

@bkabrda I tried to run ./bin/go-experimental-petstore.sh but I see a lot of changes unrelated to mine. Any advice on what I can do?

[wing328]

@arvindth please commit those and we'll review.

[arvindth]

@wing328 could you reopen this PR? I don't think the new commits on the branch will show up otherwise.

[arvindth]

After the rebase forward, ./bin/go-experimental-petstore.sh showed no changes.

[wing328]

Tests passed via https://circleci.com/gh/OpenAPITools/openapi-generator/9674

[wing328]

I'll file another PR for go-experimental generator

[wing328]

@arvindth thanks for the PR, which has been included in v4.2.0 release: https://twitter.com/oas_generator/status/1189824932345069569

[wing328]

@arvindth thanks for the PR, which has been included in v4.2.0 release: https://twitter.com/oas_generator/status/1189824932345069569