Codegen from integrations generates misleading header comment

[kggilmer]

Describe the bug

The warning at the top of codegen files specify that all files were generated by smithy-kotlin-codegen. However this is sometimes not the case, for instances when aws-kotlin-sdk, embedded integrations, and possibly in the future external integrations included at codegen time.

Expected Behavior

The codegen header should be accurate in all cases.

Current Behavior

The codegen header specifies that the smithy-kotlin-codegen module generated the file.

Possible Solution

This logic should be overridable in some way so that it can be customized via integrations. Possibly automatically.