Auto-configure an async SNS client

[chrylis]

📢 Type of change

• Bugfix
• New feature
• Enhancement
• Refactoring

📜 Description

Export the async-enabled subclass of the Amazon SNS client. Since this is built by a factory, this change is entirely declarative.

💡 Motivation and Context

fixes #564

💚 How did you test it?

The automated tests were not working because of a Checkstyle error in Core.

📝 Checklist

• I reviewed submitted code
• I added tests to verify changes
• I updated reference documentation to reflect the change
• All tests passing
• No breaking changes

🔮 Next steps

[maciejwalkowiak]

Ideally this change should be non-breaking - meaning async client should be exposed as a bean in addition to already existing sync client.

[chrylis]

Ideally this change should be non-breaking - meaning async client should be exposed as a bean in addition to already existing sync client.

The change is non-breaking as far as I can tell, but I wasn't sure whether the checklist was something I was supposed to do myself or was a code-review step.

[maciejwalkowiak]

The change is non-breaking as far as I can tell

My bad. Async one extends the Sync one.