Swift 6 preparations

[jcubit]

This PR adds actor isolation to withEvaluatorManager as done by other APIs like AsyncHTTPClient. This is explained in detail in this talk, and is part of SE-0461 where it will be the default.

I noticed this when I tried to call this function from the library I am working on. As the above SE proposal says, it's easy to miss:

It's easy to write invalid async APIs. If the performAsync method were in a library that the programmer doesn't own, it's not possible to workaround the data-race safety error without using unsafe opt outs. It's common for library authors to mistakenly vend an API like this, because the data-race safety error only manifests when calling the API from an actor.

The PR also includes a change in the swift codegen types so that they are marked Sendable, and PklSwift is imported with @preconcurrency.

In addition, marked some basic types Sendable:

• Duration
• DurationUnit
• ModuleSource
• PklEvaluatorSettings
• PklEvaluatorSettingsColor
• Http
• Proxy
• ExternalReader, and
• PathElement