Make the ReactiveResilienceStrategy type-safe

[martintmk]

Details on the issue fix or feature implementation

One reoccurring complaint about the current non-generic core was the possibility of combining incompatible reactive strategies. This PR improves the API and makes the above not possible.

This was ensured with the following changes:

• ReactiveResilienceStrategy<T> no longer derives from ResilienceStrategy.
• Added two new AddStrategy extensions for both CompositeStrategyBuilder and CompositeStrategyBuilder<T> that require factory returning ReactiveResilienceStrategy<T>.
• Reimplemented all extensions for reactive strategies to use the new API

Going forward the author of custom resilience strategies can use:

• ResilienceStrategy as a base class for non-reactive strategies
• ReactiveResilienceStrategy<T> as a base class for reactive strategies

Contributes to #1454

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

[martintmk]

cc: @vany0114

In your PR you have MonkeyStrategy<T> that derives from MonkeyStrategy. With this new API that won't be possible anymore. I was thinking of making ReactiveResilienceStrategy<T> an interface but it didn't feel right.

[codecov]

Codecov Report

Merging #1462 (cabfd57) into main (b1ec863) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1462      +/-   ##
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- Coverage   83.92%   83.89%   -0.03%     
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  Files         274      276       +2     
  Lines        6506     6495      -11     
  Branches     1012     1015       +3     
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- Hits         5460     5449      -11     
  Misses        837      837              
  Partials      209      209              

Flag	Coverage Δ
linux	83.89% <100.00%> (-0.03%)	⬇️

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Files Changed	Coverage Δ
...CircuitBreaker/CircuitBreakerResilienceStrategy.cs	100.00% <ø> (ø)
.../Polly.Core/Fallback/FallbackResilienceStrategy.cs	100.00% <ø> (ø)
...rc/Polly.Core/Hedging/HedgingResilienceStrategy.cs	100.00% <ø> (ø)
src/Polly.Core/Retry/RetryResilienceStrategy.cs	100.00% <ø> (ø)
...ircuitBreakerCompositeStrategyBuilderExtensions.cs	100.00% <100.00%> (ø)
...c/Polly.Core/CompositeStrategyBuilderExtensions.cs	100.00% <100.00%> (ø)
...back/FallbackCompositeStrategyBuilderExtensions.cs	100.00% <100.00%> (ø)
...dging/HedgingCompositeStrategyBuilderExtensions.cs	100.00% <100.00%> (ø)
src/Polly.Core/ReactiveResilienceStrategy.cs	100.00% <100.00%> (ø)
src/Polly.Core/ResilienceStrategy.cs	100.00% <100.00%> (ø)
... and 4 more