config: Design of LoadDefaultConfig options is very generic, allows invalid loading options

[jasdel]

The v2 SDK's design for LoadDefaultConfig options are very generic, and will accept any value since Config is an alias for interface{}. This means there are no direct links between the configuration loading utility and the options a user can provide. Other than looking for With... types/functions in the config module. In addition, since Config is an empty interface, users could provide any value here regardless if it is valid and usable or not.

To address these issues, I propose the config.LoadDefaultConfig function signature be changed to take function options for a LoadOptions struct instead of abstract Config empty interface type. Instead of LoadDefaultConfig taking an abstract set of configuration provider getters, LoadDefaultConfig would take functional options of a discrete set of options that are valid for loading the configuration. Helpers like WithRegion, WithSharedConfigProfile, etc would be functional options that set fields on the LoadOptions struct instead of value getters.

Usage:

cfg, err := config.LoadDefaultConfig(config.WithRegion("us-west-2"),
     func(o *config.LoadOptions) {
          o.SharedConfigProfile = "foo"
     })

config Package type definitions:

type LoadOptions struct {
     aws.Config
     
     SharedConfigProfile string
     CustomCABundle io.Reader // or string?
}

func LoadDefaultConfig(optFns ...func(*LoadOptions)) (aws.Config, error) {}

The embedded aws.Config allows configuration of default configuration options such as Region, and EndpointResolver. the value of the embedded aws.Config would be initialized with the config's current resolveDefaultConfig function before the user provided option functions are invoked. This design also removes the needed for resolvers like resolveEndpointResolvers, resolveRegion, and resolveHTTPClient.

The LoadOptions type would implement the provider getters that are used by the config loading resolvers.

[jasdel]

resolveRegion resolver would still need to exist, since region can come from additional sources, such as environment variables and shared config. In addition to ec2 region if the SDK ever supports that.

[skotambkar]

Fixed in #951

[github-actions]

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