Decode strings incrementally during async deserialization

[AArnott]

This doesn't change how we work when interning strings.
It also doesn't incrementally encode strings during async serialization. But we could add that if needed.

[codecov]

Codecov Report

Attention: Patch coverage is 95.71429% with 3 lines in your changes missing coverage. Please review.

Project coverage is 84.23%. Comparing base (9884932) to head (b01e5c3).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
...Nerdbank.MessagePack/MessagePackStreamingReader.cs	80.00%	2 Missing ⚠️
src/Nerdbank.MessagePack/SequencePool`1.cs	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #211      +/-   ##
==========================================
+ Coverage   83.82%   84.23%   +0.41%     
==========================================
  Files         157      157              
  Lines       12596    12646      +50     
  Branches     1351     1346       -5     
==========================================
+ Hits        10558    10652      +94     
+ Misses       2016     1972      -44     
  Partials       22       22              

Flag	Coverage Δ
Linux	82.29% <92.53%> (+0.35%)	⬆️
Windows	84.21% <95.71%> (+0.43%)	⬆️

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