Refactor samples.go to be more configurable

[richardm-stripe]

Reviewers

r? @vcheung-stripe
cc @stripe/developer-products

Summary

Refactors the samples command so that the code can be re-used/configured via plugin.

Specifically

• I renamed the Samples struct to SampleManager as identifier samples was referring both to the package and to an instance of Samples which I found confusing.
• I gave SampleManager a constructor helper, NewSampleManager with a default initialization, rather than having it initialized directly everywhere.
• I changed the Create and GetSampleConfig functions to be methods with a SampleManager receiver, so that they are more configurable.
• I refactored ConfigureDotEnv into SampleManager.ConfigureDotEnv (which can be given a custom implementation, now that it's on the struct), and WriteDotEnv.
• I introduced a SampleLister interface to replace getSamples, so that this can be customized (e.g., an implementation that returns a hardcoded list instead of fetching and caching from a specific github repository) and refactored the existing implementation into cachedGithubSampleLister.

Testing

As a basic sanity check, I manually ran stripe samples list and stripe samples create and they appear still to work.

[gracegoo-stripe]

Thanks for refactoring this, LGTM!