Pooling of CancellationTokenSources

[martintmk]

Details on the issue fix or feature implementation

This PR adds pooling of CancellationTokenSource on .NET 6.0+ runtimes.

Confirm the following

• I started this PR by branching from the head of the default branch
• I have targeted the PR to merge into the default branch
• I have included unit tests for the issue/feature
• I have successfully run a local build

[codecov-commenter]

Codecov Report

Merging #1116 (295c903) into main (97c2307) will increase coverage by 0.05%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##             main    #1116      +/-   ##
==========================================
+ Coverage   77.78%   77.83%   +0.05%     
==========================================
  Files         200      201       +1     
  Lines        4766     4777      +11     
  Branches      851      851              
==========================================
+ Hits         3707     3718      +11     
  Misses        854      854              
  Partials      205      205              

Flag	Coverage Δ
linux	77.83% <100.00%> (+0.05%)	⬆️
macos	77.83% <100.00%> (+0.05%)	⬆️
windows	77.83% <100.00%> (?)

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Impacted Files	Coverage Δ
src/Polly.Core/Utils/TimeProviderExtensions.cs	100.00% <ø> (ø)
...rc/Polly.Core/Timeout/TimeoutResilienceStrategy.cs	100.00% <100.00%> (ø)
...rc/Polly.Core/Utils/CancellationTokenSourcePool.cs	100.00% <100.00%> (ø)

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