Force to use only one retriever at the time

[thomaspoignant]

Description

⚠️ this PR include breaking changes.

In this PR I change the way to pass the Retriever in the config.
We don't have a property for each type of Retriever but only on Retriever which can manage all types of Retriever.

Why?

The main reason is to have the aws/aws-sdk-go as a dependency only if you are using the S3Retriever. With this new syntax, we don't force you to have the SDK if you are not using it.

How to migrate

If you were using HTTPRetriever, S3Retriever or GithubRetriever, the change consists only of changing the key in the config.

// Before v0.5.0
err := ffclient.Init(ffclient.Config{
    PollInterval: 3,
    HTTPRetriever: &ffClient.HTTPRetriever{
        URL:    "http://example.com/test.yaml",
    },
})

// Since v0.5.0
err := ffclient.Init(ffclient.Config{
    PollInterval: 3,
    Retriever: &ffclient.HTTPRetriever{
        URL:    "http://example.com/test.yaml",
    },
})

It is a bit different for the flag configuration, I have introduced a FileRetriever to keep the same format for all retrievers.

// Before v0.5.0
err := ffclient.Init(ffclient.Config{
    PollInterval: 3,
    LocalFile: "file-example.yaml",
})

// Since v0.5.0
err := ffclient.Init(ffclient.Config{
    PollInterval: 3,
    Retriever: &ffclient.FileRetriever{
        Path:    "file-example.yaml",
    },
})

Changes include

• Bugfix (non-breaking change that solves an issue)
• New feature (non-breaking change that adds functionality)
• Breaking changes (change that is not backward-comptible and/or changes current functionality)

Closes issue(s)

Resolve #42

Checklist

• I have tested this code
• I have added unit test to cover this code
• I have updated the Readme
• I have followed the contributing guide

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