Conform EventLoopFuture/Promise to Sendable

[adam-fowler]

Conform EventLoopFuture and EventLoopPromise to Sendable

Motivation:

Fixes compile warnings

Based off discussion in #1928

[fabianfett]

I'm afraid we will need to do this dance to make 5.2 - 5.4 happy:

#if swift(>=5.5)
public typealias NIOSendable = Swift.Sendable
#else 
public typealias NIOSendable = Any
#endif

Then NIO types need to conform to NIOSendable instead of Sendable.

[adam-fowler]

I'm afraid we will need to do this dance to make 5.2 - 5.4 happy:

#if swift(>=5.5)
public typealias NIOSendable = Swift.Sendable
#else 
public typealias NIOSendable = Any
#endif

Then NIO types need to conform to NIOSendable instead of Sendable.

Yeah forgot about that. I have duplicated the code from the original PR, which adds the conformance in a separate extension but yeah maybe implementing NIOSendable is the way to go as in the future there will be a ton of these.

[Lukasa]

I think we should do NIOSendable. Better to do it right the first time.

[adam-fowler]

We'll still have to stick #if swift(>=5.5) around the EventLoopFuture extension as @unchecked is only available in swift 5.5.
I'll add the typealias still. Do you want it in AsyncAwaitSupport.swift?

[Lukasa]

Nah, it'll have to go into NIOCore.

[adam-fowler]

Nah, it'll have to go into NIOCore.

AsyncAwaitSupport.swift is in NIOCore

[Lukasa]

LGTM.