Expose Randomizer property and use it in retry strategy

[martintmk]

Details on the issue fix or feature implementation

This PR exposes ResilienceStrategyBuilder.Randomizer (altought hidden for now) that allows to control the randomization behavior in tests. Additionally, I have fixed the copy constructor which omitted some properties.

This change also allows (with some tricks) to calculate the aggregated delays of retry strategy.

I will use this here instead of custom code:
https://github.com/dotnet/extensions/blob/52ebc1b123613254f9dfe250515027c1332fba35/src/Libraries/Microsoft.Extensions.Http.Resilience/Resilience/Internal/Validators/ValidationHelper.cs#L13

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]

Codecov Report

Merging #1346 (ec38ca9) into main (6b38850) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1346      +/-   ##
==========================================
+ Coverage   83.58%   83.59%   +0.01%     
==========================================
  Files         270      270              
  Lines        6389     6396       +7     
  Branches     1008     1008              
==========================================
+ Hits         5340     5347       +7     
  Misses        840      840              
  Partials      209      209              

Flag	Coverage Δ
linux	83.59% <100.00%> (+0.01%)	⬆️
macos	83.59% <100.00%> (+0.01%)	⬆️
windows	83.59% <100.00%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
src/Polly.Core/ResilienceStrategyBuilderBase.cs	100.00% <100.00%> (ø)
src/Polly.Core/ResilienceStrategyBuilderContext.cs	100.00% <100.00%> (ø)
src/Polly.Core/Retry/RetryHelper.cs	100.00% <100.00%> (ø)
src/Polly.Core/Retry/RetryResilienceStrategy.cs	100.00% <100.00%> (ø)
.../Retry/RetryResilienceStrategyBuilderExtensions.cs	100.00% <100.00%> (ø)