Introduce OutcomeEvent

[martintmk]

The issue or feature being addressed

Closes #1094 and contributes to #1093.

Details on the issue fix or feature implementation

This PR introduces the final set of event callbacks. These are void-based events that occur when something important happens. The user can register multiple callbacks for each result type.

var options = new RetryStrategyOptions();

options.OnRetry
    .Add<int>(() => Console.WriteLine("Retry occurred!"))
    .Add<int>((outcome, args) => Console.WriteLine($"Retry occurred again! Outcome: {outcome}"))
    .Add<string>(() => Console.WriteLine("Retry occurred for string-based results!"));

After this one, everything will be ready for retry strategy.

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 #1096 (c21d536) into main (b1ffcb9) will increase coverage by 0.22%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##             main    #1096      +/-   ##
==========================================
+ Coverage   76.02%   76.25%   +0.22%     
==========================================
  Files         182      185       +3     
  Lines        4418     4459      +41     
  Branches      814      821       +7     
==========================================
+ Hits         3359     3400      +41     
  Misses        854      854              
  Partials      205      205              

Flag	Coverage Δ
linux	76.25% <100.00%> (+0.22%)	⬆️
macos	76.25% <100.00%> (+0.22%)	⬆️
windows	?

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

Impacted Files	Coverage Δ
src/Polly.Core/Retry/OnRetryArguments.cs	100.00% <100.00%> (ø)
src/Polly.Core/Retry/RetryStrategyOptions.cs	100.00% <100.00%> (ø)
src/Polly.Core/Strategy/OutcomeEvent.Handler.cs	100.00% <100.00%> (ø)
src/Polly.Core/Strategy/OutcomeEvent.cs	100.00% <100.00%> (ø)
src/Polly.Core/Strategy/SimpleEvent.cs	100.00% <100.00%> (ø)

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