Change WordPressAPI and WordPressApiCache from actor to class

[crazytonyli]

Considering WordPressAPI and WordPressApiCache do not have mutable states, and all their properties conform to Sendable, declaring them as a Sendable class should have no practical impact.

The main motivation is that I don't think we need/should use await when accessing their properties:

let api: WordPressAPI = ...
let delegate = /* ~await~ */ api.apiClientDelegate

let apiCache: WordPressApiCache = ...
let cache = /* ~await~ */ apiCache.cache

use(delegate)

BTW, the publicized properties help integrate #1024 into the app.

[jkmassel]

What's the reason for this change? I think I'd rather define whatever methods we need on WordPressAPI than allow others to reach into this objects implementation?

[crazytonyli]

This is to provide a convenient way to pass the delegate instance to WpSelfHostedService. I have reverted this change. There are other ways, though less convenient.