Cleanup of callback API

[martintmk]

Details on the issue fix or feature implementation

• Turning events, predicates and generators from abstract to sealed classes. The extra complexity didn't warrant the added value.
• Rename SimpleEvent to NoOutcomeEvent
• Added non-generic methods to OutcomeGenerator. This allows adding new extensions for retry.
• Adding a new generic OutcomePredicate<TArgs, TResult> that only configures the predicates for a specific result-type.
• Deleting unnecessary classes.
• Prefixing methods in OutcomePredicate with Handler
• Renaming methods in OutcomeEvent from Add to Register.

In the follow-up PR I will also add non-generic overloads to OutcomeEvent

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 #1125 (284802b) into main (8b944d3) will increase coverage by 0.26%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##             main    #1125      +/-   ##
==========================================
+ Coverage   77.88%   78.14%   +0.26%     
==========================================
  Files         202      201       -1     
  Lines        4788     4846      +58     
  Branches      858      863       +5     
==========================================
+ Hits         3729     3787      +58     
  Misses        854      854              
  Partials      205      205              

Flag	Coverage Δ
linux	78.14% <100.00%> (+0.26%)	⬆️
macos	?
windows	78.14% <100.00%> (+0.26%)	⬆️

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% <ø> (ø)
src/Polly.Core/Retry/RetryDelayArguments.cs	100.00% <ø> (ø)
src/Polly.Core/Retry/ShouldRetryArguments.cs	100.00% <ø> (ø)
src/Polly.Core/Strategy/OutcomeEvent.Handler.cs	100.00% <ø> (ø)
...lly.RateLimiting/OnRateLimiterRejectedArguments.cs	100.00% <ø> (ø)
src/Polly.Core/Retry/RetryResilienceStrategy.cs	100.00% <100.00%> (ø)
.../Retry/RetryResilienceStrategyBuilderExtensions.cs	100.00% <100.00%> (ø)
src/Polly.Core/Retry/RetryStrategyOptions.cs	100.00% <100.00%> (ø)
src/Polly.Core/Strategy/NoOutcomeEvent.cs	100.00% <100.00%> (ø)
src/Polly.Core/Strategy/OutcomeEvent.cs	100.00% <100.00%> (ø)
... and 10 more

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