Deprecate service.awsKmsKeyArn in favor of provider.kmsKeyArn

[medikoo]

Use case description

There's lone AWS provider property to be setup in context of service while all others are set in context of provider.

Historically (#3672 (comment)) it looks it was added as a start of moving everything from provider scope to service scope. Still It was not followed, and I don't see a reason, now, why configuration as service.aws* is better than provider.*

I believe, that for consistency we should move this property to provider context. It also makes schema configuration for AWS provider more complex.

Proposed solution

1. Introduce provider.kmsKeyArn (it should shadow service.awsKmsKeyArn, but functions[].kmsKeyArn should take precedence)
2. Deprecate service.awsKmsKeyArn usage
3. Introduce functions[].kmsKeyArn (for consistency) and deprecate functions[].awsKmsKeyArn

Implementation should take place here:

serverless/lib/plugins/aws/package/compile/functions/index.js

Lines 257 to 265 in 29d5fe7

	let kmsKeyArn;
	const serviceObj = this.serverless.service.serviceObject;
	if ('awsKmsKeyArn' in functionObject) {
	kmsKeyArn = functionObject.awsKmsKeyArn;
	} else if (serviceObj && 'awsKmsKeyArn' in serviceObj) {
	kmsKeyArn = serviceObj.awsKmsKeyArn;
	}
	if (kmsKeyArn) {

Also changes should be reflected in config schema (however schema should maintain support for deprecated properties)

[medikoo]

Covered by #8277