Mark return value of .asTypeOf as "deprecated read-only" (backport #4198)

[mergify]

Users will now get a deprecation warning if they connect to the result of .asTypeOf.

Marked for backporting to 6.x so that we can change this in 7.0.

Contributor Checklist

• Did you add Scaladoc to every public function/method?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
• Did you specify the type of improvement?
• Did you add appropriate documentation in docs/src?
• Did you request a desired merge strategy?
• Did you add text to be included in the Release Notes for this change?

Type of Improvement

• API deprecation

Desired Merge Strategy

• Squash

Release Notes

Previously, .asTypeOf would return a Wire. To get the old behavior, wrap the .asTypeOf call in WireInit(...).

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels? (Select the most appropriate one based on the "Type of Improvement")
• Did you mark the proper milestone (Bug fix: 3.6.x, 5.x, or 6.x depending on impact, API modification or big change: 7.0)?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you do one of the following when ready to merge:
  • Squash: You/ the contributor Enable auto-merge (squash), clean up the commit message, and label with Please Merge.
  • Merge: Ensure that contributor has cleaned up their commit history, then merge with Create a merge commit.

---

This is an automatic backport of pull request #4198 done by [Mergify](https://mergify.com).