Add new custom-resource patching mechanism for serverless>3.39.0

[dfangl]

Motivation

With 3.39.0, serverless migrated their custom resource lambedas to AWS SDK v3, which broke our patching of the s3ForcePathStyle into the global AWS config object.

252 explains the issue in more detail, with impact explained.

We need to adapt our patching mechanism, while still maintaining backwards compatibility with older serverless versions.

Changes

• Add new sdk-v3 patching mechanism. There is no global config object anymore, so we are replacing the sdk client instantiation.
• Try the old patching first, if line is not found, use the new mechanism. This should maintain backwards compatibility.

[dfangl]

I also updated the version to 1.2.1 and added it in the readme. Would be nice if we could release that version after merge