Introduce ResilienceContextPool (ApiReview)

[martintmk]

Details on the issue fix or feature implementation

Contributes to #1365

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

[martincostello]

Should this be a property rather than a field?

[codecov]

Codecov Report

Merging #1421 (75e29eb) into main (02a1944) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1421      +/-   ##
==========================================
- Coverage   83.75%   83.74%   -0.01%     
==========================================
  Files         274      276       +2     
  Lines        6456     6454       -2     
  Branches     1007     1007              
==========================================
- Hits         5407     5405       -2     
  Misses        840      840              
  Partials      209      209              

Flag	Coverage Δ
linux	83.74% <100.00%> (-0.01%)	⬇️
macos	83.74% <100.00%> (-0.01%)	⬇️
windows	83.74% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...Core/CircuitBreaker/CircuitBreakerManualControl.cs	100.00% <100.00%> (ø)
src/Polly.Core/Hedging/Controller/TaskExecution.cs	100.00% <100.00%> (ø)
src/Polly.Core/ResilienceContext.cs	100.00% <100.00%> (ø)
src/Polly.Core/ResilienceContextPool.Shared.cs	100.00% <100.00%> (ø)
src/Polly.Core/ResilienceContextPool.cs	100.00% <100.00%> (ø)
...c/Polly.Core/ResilienceStrategy.Async.ValueTask.cs	100.00% <100.00%> (ø)
.../Polly.Core/ResilienceStrategy.Async.ValueTaskT.cs	100.00% <100.00%> (ø)
src/Polly.Core/ResilienceStrategy.Sync.cs	100.00% <100.00%> (ø)
src/Polly.Core/ResilienceStrategy.SyncT.cs	100.00% <100.00%> (ø)
src/Polly.Core/ResilienceStrategy.cs	100.00% <100.00%> (ø)
... and 2 more