Add support for overriding host

[tfwright]

#49

Notes:

• I added support for overriding the entire host rather than specifically skipping the prefix as it seemed much more flexible for similar cost.
• Not tested. I'm not sure what the preferred way to add tests for the template code is.
• My first time touching Erlang code. Let me know what I screwed up :)
• PRs for client code allowing specification of host field to follow

[jfacorro]

@tfwright Thank you for the contribution 😄 . There's currently a lack of testing so for the time being it would be helpful and preferable to accompany a PR in the generator with a link to the generated aws_erlang code and aws-elixir code. Let me know if you need assistance with generating these.

[tfwright]

@jfacorro happy to generate those, but where should the links I would add in the PR point to? PRs on those repos?

[jfacorro]

@tfwright I have been mainly doing what you describe. Opening PRs in the generated repos so that the pipelines there get triggered. It's a little awkward but it's only until we include the generating and building of those in this repos pipeline.

[josevalim]

@jfacorro another option is to bundle the three into a single git repo. Both rebar3 and mix support sparse checkouts, so people can still access the lib individually.

[josevalim]

@tfwright how would you setup the host from the Elixir side? We want to add some tests with bypass and it may require this.

[tfwright]

@josevalim it would be a new option key in the client struct that would preempt the keys currently used to compose the host (prefix, region, endpoint)

[sherbondy]

This is great! Something like this is also necessary to support AWS's Elemental MediaConvert service.
For whatever reason, it has customer-specific endpoints with a per-account 8-digit prefix:
https://docs.aws.amazon.com/mediaconvert/latest/apireference/getting-started.html

[onno-vos-dev]

I'll take a look and pick this up in the coming week or so 👌