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kotlinlang

Hi, I'm trying to add `@EncodeDefault` for kotlinx.serialization and have opened a PR to implement this feature experimentally. (<https://github.com/Kotlin/kotlinx.serialization/issues/1091|#1091>)
Here are some things needing more investigation:
1. Is a "Never Encode Default" mode needed? (useful to handle null to be dropped, especially for null-not-supported formats like Protobuf)
2. `@EncodeDefault(true)` `@EncodeDefault(false)` or `EncodeDefault(EncodeDefaultMode.ALWAYS)` `EncodeDefault(EncodeDefaultMode.NEVER)`, which style do you prefer?
3. Is a explicit flag to use the format's global settings needed? (`EncodeDefault(EncodeDefaultMode.DEFAULT)`)