[SNS] - Refactor SNSTemplate test to use LocalstackContainer

[MatejNedic]

In spring-cloud-aws-sns module tests in class SnsTemplateTest currently are not using testcontainers and loclastack for testing.

For reference check SnsBatchTemplateTest.

[haroya01]

In the SNS test code, should the configuration-related code be extracted into an interface like we did for the S3 test code?
If that’s correct, may I go ahead and work on it?

[MatejNedic]

Tnx @haroya01 , assigned you to issue.

If you need anything don't hesitate to write. I have filled description of the issue what needs to be done.

[haroya01]

#1580

Since this is my first contribution, I’m not sure if I submitted it correctly. Could you please review it and let me know if everything looks okay?