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

[sebsto]

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

Swift feature flags

Enables four feature flags in Package.swift:

• NonisolatedNonsendingByDefault
• ExistentialAny
• MemberImportVisibility
• InternalImportsByDefault

Warnings are treated as errors via treatAllWarnings(as: .error). To make it compile: added any to existential types, marked imports as public import where a dependency's types appear in public API, and added a few missing Foundation imports in tests.

The Soto clients under GeneratedClients/ are generated, so scripts/generate-aws-clients.sh now emits any and public import automatically. Two AWSErrorShape.Type uses can't take any yet (soto-core hasn't adopted it), so they stay as warnings via treatWarning("ExistentialAny", as: .warning).

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
    // ...
}

Runtime entry points now default their logger: parameter to Logger.current. Since the project enables NonisolatedNonsendingByDefault, the binding is active across all supported toolchains (6.2 to 6.4). Task-local values propagate through structured concurrency but are not inherited by Task.detached. See the new logging.md article for details.

Testing

Tested with swift build and swift test. CI covers Swift 6.2, 6.3, and 6.4 on Linux.