feat(ios): CAPPluginMethod selector-based initializer

[Steven0351]

Add secondary initializer for CAPPluginMethod that takes a selector. Creates a convenience initializer as well with a Swift enum to avoid having to rely on global string values in the style of CAPPluginReturnPromise. For a pure Swift plugin implementation, it makes the definition potentially go from:

class MyPlugin: CAPPlugin, CAPBridgedPlugin {
  let pluginMethods: [CAPPluginMethod] = [
    .init(name: "promiseMethod", returnType: CAPPluginReturnPromise),
    .init(name: "callbackMethod", returnType: CAPPluginReturnCallback)
  ]

  @objc func promiseMethod(_ call: CAPPluginCall) {}
  @objc func callbackMethod(_ call: CAPPluginCall) {}
}

to this:

class MyPlugin: CAPPlugin, CAPBridgedPlugin {
  let pluginMethods: [CAPPluginMethod] = [
    .init(#selector(promiseMethod)),
    .init(#selector(callbackMethod), returnType: .callback)
  ]

  @objc func promiseMethod(_ call: CAPPluginCall) {}
  @objc func callbackMethod(_ call: CAPPluginCall) {}
}

A benefit of this approach include removing the need to stringify the name of the function. This will also validate the selector at compile time, so if it has not been annotated with @objc it will be a compiler error.

[jcesarmobile]

should we add the none return type?

it's not used by any plugin other than console (as far as I know)

[Steven0351]

We definitely could. none is weird in that the web code treats anything that is not a promise as a callback https://github.com/ionic-team/capacitor/blob/main/core/src/runtime.ts#L123-L133. The docs are actually misleading https://capacitorjs.com/docs/plugins/method-types#void-return in that it says "You can check the promise for an error but when it resolves the result is ignored" because you can't actually check for an error.

[jcesarmobile]

let's add it, so if we want to do a refactor in the future for the core plugins, we can use this new initializer in all of them instead of having to use a different one for Console plugin