Support serializing index-based types as maps as well as arrays

[AArnott]

Arrays are most compact when they are packed with data. But if there are 'holes' in the indexes (whether by missing indexes in the key attributes or by properties having default values and thus not serialized), a map that uses those same ordinal indexes as keys can be more compact.
With this new feature, the more compact of the two formats is automatically chosen (when SerializeDefaultValues == false) during serialization.

[codecov]

Codecov Report

Attention: Patch coverage is 96.97733% with 12 lines in your changes missing coverage. Please review.

Project coverage is 77.83%. Comparing base (39101a7) to head (29d97d3).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
...k.MessagePack/Converters/ObjectArrayConverter`1.cs	94.55%	3 Missing and 5 partials ⚠️
src/Nerdbank.MessagePack/MessagePackAsyncReader.cs	75.00%	1 Missing and 1 partial ⚠️
...Nerdbank.MessagePack.Tests/ObjectsAsArraysTests.cs	98.90%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #112      +/-   ##
==========================================
+ Coverage   76.57%   77.83%   +1.26%     
==========================================
  Files         127      127              
  Lines        7593     7914     +321     
  Branches     1048     1101      +53     
==========================================
+ Hits         5814     6160     +346     
+ Misses       1448     1425      -23     
+ Partials      331      329       -2     

Flag	Coverage Δ
Linux	77.83% <96.97%> (+1.26%)	⬆️

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