Audit all public enums

[weissi]

Having public enums is a major burden if you’re API stable and you don’t use the “library evolution” Swift language dialect. SwiftNIO (like all SwiftPM packages) uses the non-lib-evolution dialect and therefore nothing in enums can ever be changed.

This has caused us many issues and for NIO 3 we should audit all public enums and probably make most of them structs with static constructors.

See also:

• Add WebSocket close code 1005 and 1006. #1410
• ChannelShouldQuiesceEvent should go into the ChannelEvent enum #1014
• make all error enums extensible #969