Allow to set SNS Message Attributes via context

[taraspos]

The suggested fix for issue #131

[jprobinson]

Howdy @trane9991! Thanks for contributing!

I like the idea of using the context to pass message attributes, but I wonder if we can do something to clean up the way the key is used for the context values.

You could make a private key type/variable and then provide a new function (something likeWithMessaggeAttributes(context.Context, map[string]*sns.MessageAttributeValue) context.Context maybe?) to add those attributes to the context. Then the Publish func would similarly grab those attributes, but they'd use the private key instead of a string.

Here's an example from the Go blog: https://blog.golang.org/context#TOC_3.2.

[taraspos]

@jprobinson thanks for the suggestion. Done.

[jprobinson]

🌯 🔥 ! thanks!

[jprobinson]

This change has been tagged in https://github.com/nytimes/gizmo/releases/tag/v1.3.2