Rule request: [await_within_actor] actor re-entrancy problem may cause a subtle bug

New Issue Checklist

• Updated SwiftLint to the latest version
• I searched for existing GitHub issues

New rule request

Please describe the rule idea:
[await_within_actor] Awaiting within an actor may cause subtle bugs

Actor state may change during suspension points and assumptions made before need to be verified afterwards.

Another approach is to avoid accessing mutable state before an await within a function call or restructuring logic:
https://swiftsenpai.com/swift/actor-reentrancy-problem/

1. Why should this rule be added?
   Awaiting within an actor may cause subtle bugs as shown in the link below
   https://www.wwdcnotes.com/notes/wwdc21/10133/

2. Example - trigger:

actor ImageDownloader {
  private var cache: [URL: Image] = [:]
  func image(from url: URL) async throws -> Image? {
    if let cached = cache[url] { return cached }
    let image = try await downloadImage(from: url) // Trigger
    cache[url] = image // 👈🏻 Potential bug: `cache` may have changed
    return image
  }
}

4. Should the rule be configurable, if so what parameters should be configurable?
   Not at this time.

5. Should the rule be opt-in or enabled by default? Why?
   Opt-in because this may be difficult to guard against false positives as there are many patterns to protect against the 'actor re-entrancy problem'.

Start with the actor re-entrancy example in the issue and the linked Swift concurrency references; no implementation files or tests are identified. Done means the opt-in [await_within_actor] rule's behavior and false-positive boundaries are agreed and verified against representative examples.