Add support for http(s) proxy in AWS provider

[dkronst]

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Description

In some setups, an http proxy is used (e.g. for security reasons). It, therefor, may be desirable to have the proxy set as part of the provider's configuration (and not, for example, in the environment - since it will affect the entire process and not just specific configurations).
To support this configuration option in the provider code, support for http proxy has to first be added to the aws sdk go wrapper.

Potential Library Implementation

For example:

	if c.HTTPProxy != "" {
		proxyUrl, err := url.Parse(c.HTTPProxy)
		if err != nil {
			return nil, err
		}

		transport.Proxy = http.ProxyURL(proxyUrl)
	}

Potential Terraform Backend/Provider Configuration

provider "aws" {
   http_proxy = "http://my_proxy:8080"
   region = "cn-northwest-1"
}

References

• Add http proxy support as a config option #46
• Support for HTTP proxy in provider configuration terraform-provider-aws#14942

[gdavison]

Closed by merging #46