Refactoring Config loading, replacing the need for a Session.

[jasdel]

Adds an external package to load external configuration data to the SDK. This replaces the loading work performed by the session package. In addition this will remove the need for a session and the SDK can remove this concept.

The new external config loading package is based on modular components that are plugged together to load external configuration, and resolve that configuration into an aws.Config value. This allows customers the flexibility to configure the SDK as they need.

This does not support service specific external configuration. It is not defined how or if this will be supported in the future.

Load default configuration without any customization or options:

cfg, err := external.LoadDefaultAWSConfig()
if err != nil {
    panic(fmt.Sprintf("failed loading config, %v", err)
}

Load default configuration while also providing a shared config profile
in addition to the default shared config and env config.

extCfgs, err := external.Config{
    StaticSharedConfigProfile("test-profile"),
}.AppendFromLoaders(DefaultConfigLoaders)
if err != nil {
    panic(fmt.Sprintf("failed loading config, %v", err)
}

cfg, err := extCfgs.ResolveAWSConfig(DefaultAWSConfigResolvers)
if err != nil {
    panic(fmt.Sprintf("failed loading config, %v", err)
}

TODO:

• Consider the simple use case of SharedConfig profile as being too verbose, helper maybe?
• Add tests for external package.
• Remove Session (Add handlers to aws.Config).
• Shoudl add additional helpers WithSharedConfigProfile("test-profile"), WithEndpointResolver(StaticEndpoint("abc")), WithRegion("blah"), or is this complicating the loader with AWSConfig specific values.?

[jasdel]

One possible way to solve the shared config usability issue is to update LoadDefaultAWSConfig() and allow it to take a variadic list of external.Config that will seed the external.Configs.

Signature update:

func LoadDefaultAWSConfig(configs ...Config) (aws.Config, error) {}

Updated Usage:

cfg, err := external.LoadDefaultAWSConfig(
    WithSharedConfigProfile("test-profile"),
)
if err != nil {
    panic(fmt.Sprintf("failed loading config, %v", err))
}

[jasdel]

dropping this PR creating PR against feature branch.