fix: missing export for api key auth middleware

[eduardomourar]

Issue #, if available: N/A

Description of changes:

This fix will make easier to use the HttpApiKeyAuth middleware by exporting it in the entrypoint of a NPM package. That way we can simply import like this:

import { getHttpApiKeyAuthPlugin } from "sample-client-sdk";

Instead of:

import { getHttpApiKeyAuthPlugin } from "sample-client-sdk/dist-types/middleware/HttpApiKeyAuth";

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[eduardomourar]

@adamthom-amzn , could you help us with this PR, please?

[adamthom-amzn]

I'm not familiar with the use of this middleware - is the middleware not added automatically when the model has the trait applied? Do you have an example of why you would need to import something from this middleware in your code?

[eduardomourar]

It only adds automatically the configuration for me, not the auth middleware.

Also, I would like to make some customization to it (for instance, use it to populate a bearer token in the authorization header), so I have to use the middleware being autogenerated because there is no public library like this one.