Modernize the codebase for Swift 6.4 and beyond, groundwork for v3

[sebsto]

Modernizes the codebase (still builds on 6.2, 6.3, and 6.4), remove deprecated functions, and lays the groundwork for v3.

Swift feature flags

Enables four feature flags in Package.swift:

• NonisolatedNonsendingByDefault
• ExistentialAny
• MemberImportVisibility
• InternalImportsByDefault

Task-local logger

The runtime binds the per-invocation Logger (carrying requestID / traceID) as the task-local Logger.current for the duration of the handler. Any code in the call tree can read it and inherit the request metadata, with no need to thread context.logger through every function:

import AWSLambdaRuntime
import Logging

func validate(_ event: Request) {
    Logger.current.debug("Validating request")  // carries this invocation's requestID / traceID
}

let runtime = LambdaRuntime { (event: Request, context: LambdaContext) in
    validate(event)
    context.logger.info("Processing request")   // context.logger and Logger.current are equivalent here
    // ...
}

See the new logging.md article for details.