Export the resources defined in extensions_api_client.go

[ae-ou]

Is your feature request related to a problem? Please describe.
In order for a Lambda Extension to work, you need to register it with the Extensions API during the Lambda's Init phase.

The AWS docs show how to make the API call to register an extension, and they provide an OpenAPI document for the Extensions API, but you would either have to generate Go bindings from this document, or you would have to [manually] write an implementation based on it.

This repo already contains an Extensions API client (for the register and next endpoints), but all of the resources within this package are unexported.

Describe the solution you'd like
I would like to export the resources inside of the extensions_api_client.go file - this would mean that developers who wish to author Lambda Extensions would have a common client to use.

Describe alternatives you've considered
I considered generating client bindings using the OpenAPI document that AWS provides in their documentation, but this file isn't versioned, and there's no way to be notified of an update to the file.

Additional context
There are additional paths/methods on the Extensions API that aren't defined within this repo (/extension/init/error and /extension/exit/error).