feat!: Make Retryer conform to a public interface

[jbelkins]

Issue #

awslabs/aws-sdk-swift#699

Description of changes

Configuration objects may now define properties whose type is different from the concrete type. This is used to provide an abstract Retryer while creating it from a conforming concrete type.

• The type field on ConfigField is replaced by a concreteType and a protocolType. protocolType is optional, and if provided, is used as the type of the config property. If protocolType is not provided, the config property is of the same type as the concrete value.

The Retryer protocol is exposed publicly to define an interface for controlling retry.

• The existing Retryer protocol type is made public.
• RetryerMiddleware is configured with an abstract Retryer instead of a concrete SDKRetryer, to allow different retry mechanisms to be easily used with the same middleware.
• The SDKRuntimeConfiguration protocol provides a Retryer instead of a concrete SDKRetryer.
• The DefaultSDKRuntimeConfiguration is modified to conform to SDKRuntimeConfiguration.

Scope

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[jbelkins]

Closing, this will be performed in #533 instead.