Add `indirect` keyword to enums that have associated values that reference itself

[epau]

Currently, it's possible for code gen to create recursive enums (enums that have associated values that reference themselves).
We should add the indirect keyword to these enum definitions as prescribed by the swift language https://docs.swift.org/swift-book/LanguageGuide/Enumerations.html

[epau]

Closing in favor of awslabs/aws-sdk-swift#712