(@aws-cdk/aws-lambda): add an object version parameter to CfnParametersCode

[alexnikolis]

The CfnParametersCode class currently adds a bucketName and objectKey parameter to the stack. I would like to add objectVersion as well.

Use Case

I am concerned about a scenario where a hotfix needs to be made to the CloudFormation template that should not include an update to the Function's code, but the s3 code archive has been update separately.

Since the object version has not been pinned, I believe this would result in a new version being created and an unintentional update of the lambda's code.

Proposed Solution

A rough implementation was discussed in the original PR:
#2027 (comment)

Essentially just add the objectVersion parameter following the same convention as the bucket and key parameters.

Other

The existing CfnParametersCode implementation:
https://github.com/aws/aws-cdk/blob/master/packages/@aws-cdk/aws-lambda/lib/code.ts#L248

• 👋 I may be able to implement this feature request
• ⚠️ This feature might incur a breaking change

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This is a 🚀 Feature Request

[alexnikolis]

Another thought I had on this - fn.currentVersion is currently only supported for functions that use lambda.Code.fromAsset or lambda.Code.fromInline. This is problematic since addVersion is depreciated now, so continuous deployment for a Function using external Code seems to require using the latestVersion property, which has a number of disadvantages.

I believe an object version could be used to automatically generate a new lambda version for currentVersion.

https://docs.aws.amazon.com/cdk/api/latest/docs/aws-lambda-readme.html#versions-and-aliases
https://docs.aws.amazon.com/cdk/latest/guide/codepipeline_example.html#codepipeline_example_lambda

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