[Akka.Actor][Perf] Reduce Envelope copying in message-processing hotpaths

[Aaronontheweb]

Changes

Depends on #6829

While working on some performance fixes for Phobos we noticed some significant improvements by passing all structs handled inside a tight loop as in parameters into subsequent methods - this helps reduce message copying by allowing the "lower stack" to reference the memory previously allocated in the "upper stack"

Since this handling is all synchronous, there's no cost to using in parameters in these areas - and we saw a significant upside in processing performance, as much as 10% in a single instance so far.

Based on that work, I decided to give that a try with how we handle the Envelope payloads inside Akka.Actor - as these are also structs that are passed around in tight loops.

This PR introduces some API changes but these should all be binary compatible, minus any libraries that intentionally override these methods - those will need to be updated to also include the in parameter on those methods.

Checklist

For significant changes, please ensure that the following have been completed (delete if not relevant):

• This change follows the Akka.NET API Compatibility Guidelines.
• This change follows the Akka.NET Wire Compatibility Guidelines.
• I have reviewed my own pull request.
• Changes in public API reviewed, if any.

Latest dev Benchmarks

From the PingPong benchmark inside Akka.Benchmarks:

BenchmarkDotNet=v0.13.2, OS=Windows 11 (10.0.22621.1848)
12th Gen Intel Core i7-1260P, 1 CPU, 16 logical and 12 physical cores
.NET SDK=7.0.302
  [Host]     : .NET 7.0.5 (7.0.523.17405), X64 RyuJIT AVX2
  Job-VSSPYZ : .NET 7.0.5 (7.0.523.17405), X64 RyuJIT AVX2

InvocationCount=1  IterationCount=10  LaunchCount=10  
RunStrategy=Monitoring  UnrollFactor=1  WarmupCount=10  

Method	Mean	Error	StdDev	Req/sec
Actor_ping_pong_single_pair_in_memory	244.2 ns	1.75 ns	5.15 ns	4,095,040.09

This PR's Benchmarks

BenchmarkDotNet=v0.13.2, OS=Windows 11 (10.0.22621.1848)
12th Gen Intel Core i7-1260P, 1 CPU, 16 logical and 12 physical cores
.NET SDK=7.0.302
  [Host]     : .NET 7.0.5 (7.0.523.17405), X64 RyuJIT AVX2
  Job-HBAPGK : .NET 7.0.5 (7.0.523.17405), X64 RyuJIT AVX2

InvocationCount=1  IterationCount=10  LaunchCount=10  
RunStrategy=Monitoring  UnrollFactor=1  WarmupCount=10  

Method	Mean	Error	StdDev	Req/sec
Actor_ping_pong_single_pair_in_memory	242.6 ns	1.60 ns	4.71 ns	4,122,578.29

IMHO - kind of looks like the benefits of this fall within the margin of error.