Concurrency adjustments, SPM: platforms, Demo: use LOCAL Package

[Sajjon]

Several improvements to the SDK and minor adjustments to the demo app.

platforms SPM settings

Use platforms settings in Package.swift - this allows us to drop #available checks in code. Rationale: it is better to fail at compile time for developers using the Package instead of failing at runtime for users.

Use local SPM package in Demo app instead of remote.

I've change so that the Demo app now uses the local Package instead of a remote. This is the intended usage (or should be!), allowing us to test our unmerged changes. This requires the DfnsDemo/Package.swift trick.

(Pre)Concurrency fixes

The codebase is not at all prepared for strict concurrency checking / Swift Language Version: 6. So I've fixed the low hanging fruit:

• Package: Marked all structs in DfnsApi namespace as Sendable
• Package: Marked required closures @Sendable and all in Sources/DfnsSdk/Imported/Passkey.swift
• DemoApp: Marked import of DfnsSdk with @preconcurrency to silence some concurrency warnings
• DemoApp: Ensure Strict Concurrency Checking is set to Minimal for all targets (project level)
• DemoApp: Ensure Swift Language Version is set to 5 for all targets (project level)

Swift best practices

"Closed" the namespace enum DfnsApi directly (enum DfnsApi {}) making it clear it is just a namespace enum You should consider removing it, you can already do DfnsSdk.AtypeInThePackage when consumed in e.g. Demo app. These kind of namespaces are common on feature level, but not really needed on package level. Furthermore, if any symbols do conflict there are other ways to disambiguate, e.g. if conflicting with Swift Foundation types one can do e.g. Foundation.Data and ultimately SE-0491 will allow for full disambiguation,

In Swift it is best practice to put the initialiser below the stored properties of a type, which I've changed to.

I've also added // MARK: MyType annotations in Sources/DfnsSdk/DfnsApi.swift for best minimap rendering (see minimap on right hand side):

This also allows for usage of "jump bar" (CTRL 6) allowing us to jump to symbols within a large file

[Sajjon]

Removed from the target DfnsDemo and moved to the DfnsDemo project (thus covering ALL targets, e.g. unit test targets when/if added).

(You will see two of these guys, one for Debug and one for Release.)

[Sajjon]

Git made a poor rendering of this diff.... I've removed #available checks, which are not needed thanks to newly added:

platforms: [
	.macOS(.v10_15),
	.iOS(.v15),
	.watchOS(.v6),
	.tvOS(.v13)
],