[extension/awsmiddleware] Add ID to context to trace requests.

[jefchien]

Description: Previously, there was no way to determine if a received request/response were a pair. Adds an ID to the context in the RequestHandler wrappers and retrieves it in the ResponseHandler wrappers. Passes the ID to the handlers so implementations can track the ID.

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Other changes

• Combines RequestHandlers() and ResponseHandlers() functions into a single Handlers() function to make it easier to create handlers that implement both and need to be configured on both.
• Changes the AWS SDKv1 response handler to ValidateResponse instead of Unmarshal. This is so the response handler will be reached on 4xx/5xx status codes. If there's an error, it will never get to Unmarshal (see aws-sdk-go requests.go). Keep in mind that if retries are enabled, ValidateResponse will be run on each attempt.
• Fixes the go.mod package name. Should have been amazon-contributing instead of open-telemetry. This broke imports.

	module declares its path as: github.com/open-telemetry/opentelemetry-collector-contrib/extension/awsmiddleware
	        but was required as: github.com/amazon-contributing/opentelemetry-collector-contrib/extension/awsmiddleware

• Changes config to an ID to match other extension implementation (e.g. StorageID and Getter function). This allows other components to simply add

Middleware *component.ID `mapstructure:"middleware,omitempty"`

to their configurations and use the awsmiddleware.GetConfigurer(host.GetExtensions(), *cfg.Middleware).

• Adds mock extension/handler/host to make it easier to test in other components.

Link to tracking Issue: N/A

Testing: Added unit tests for the requests. Imported this commit into go.mod without issue.

Documentation: Updated README to match changes.

[SaxyPandaBear]

lgtm