[Golang][client] Allow access to APIClient.cfg.HTTPClient & APIClient.cfg.BasePath

[grokify]

Description

There are scenarios where using the raw *HTTP.Client is desirable. This includes:

1. Scenarios where API definition is not currently supported by OpenAPI Generator and a fallback to native *HTTP.Client with BasePath is necessary. Examples include:

• requests with multiple multipart/form-data files. Today only one file is supported in APIClient.prepareRequest
• multipart/mixed responses. This is not handled in client.go's detectContentType

1. Scenarios where a third-party library uses a *HTTP.Client

Providing access to the *HTTP.Client and Configuration.BasePath is useful in these scenarios.

Of course, we should support the API use cases, but sometimes more invasive changes take longer to implement.

openapi-generator version

master

OpenAPI declaration file content or url

          -
            name: "attachment"
            in: "formData"
            description: "File to upload"
            required: false
            type: "array"
            items:
              type: "file"

Command line used for generation

n/a

Steps to reproduce

n/a

Related issues/PRs

Suggest a fix/enhancement

Either:

1. Provide an accessor to APIClient.cfg
2. Provide accessors to APIClient.cfg.HTTPClient and APIClient.cfg.BasePath