Refactor environment variable handling into new Sources::EnvSource

[cjlarose]

In principle, loading settings from environment variables isn't much different from loading them from a YAMLSource or a HashSource. This change unifies the handling of environment variables into the existing "sources" abstraction.

My own personal motivation for this change is essentially the same as #271. I want to load settings into config that I got from AWS Secrets Manager. Although AWS Secrets Manager's API technically allows you to store arbitrary string values, the AWS Console UI and the AWS Secrets Manager API encourages you to store your application's secret configuration as as single Secret resource that is a flat, JSON-encoded key-value mapping. This maps very nicely to how one might use environment variables to configure an application.

Editing secrets in the AWS console

Actual plaintext

The problem is that while config offers the ability to parse nested settings from ENV, it doesn't allow you to do the same for settings you got from another source, like AWS Secrets Manager. As a result, users are forced into an awkward workaround: export all secrets from AWS Secrets Manager into ENV, then ask config to load the ENV again. My proposed EnvSource allows users to skip the loading-into-ENV step and just hand the flat Hash over to config directly.

For example,

# fetch secrets from AWS
client = Aws::SecretsManager::Client.new
response = client.get_secret_value(secret_id: "#{ENV['ENVIRONMENT']}/my_application")
secrets = JSON.parse(response.secret_string)

# load secrets into config
secret_source = Config::Sources::EnvSource.new(secrets)
Settings.add_source!(secret_source)
Settings.reload!

[cjlarose]

@pkuczynski Config::Options#load_env! and Config::Options#reload_env! are not documented at all, but are technically public methods.

As such, depending on whether or not we consider all visible methods to be part of the public API of this gem, this could be considered a breaking change and warrants a major version bump, so I added this under Breaking Changes in the changelog.

The change for users that use #load_env! or #reload_env! is simple: they should just replace those usages with #reload!, which is documented.

[cjlarose]

@pkuczynski let me know if you have any concerns

[cjlarose]

Ended up speaking to @pkuczynski about this in another channel. He's cool with merging and releasing.

This change was released in 3.0.0.