Introduce Fallback Resilience Strategy

[martintmk]

The issue or feature being addressed

Closes #1097

Details on the issue fix or feature implementation

This PR adds the fallback resilience strategy to V8.

API usage:

// convenience fallback that handles single result type
new ResilienceStrategyBuilder().AddFallback<int>(
    shouldHandle => shouldHandle
        .HandleResult(-1)
        .HandleException<InvalidOperationException>(),
    (outcome, context) => new ValueTask<int>(0));

// fallback that handles single result type
new ResilienceStrategyBuilder().AddFallback(new FallbackStrategyOptions<int>
{
    ShouldHandle = new OutcomePredicate<HandleFallbackArguments, int>()
        .HandleResult(-1)
        .HandleException<InvalidOperationException>(),
    FallbackAction = (outcome, context) => new ValueTask<int>(0),
    OnFallback = new OutcomeEvent<OnFallbackArguments, int>().Register(() => Console.WriteLine("Fallback"))
});

// the main method allowing configuration of fallbacks for multiple result types
new ResilienceStrategyBuilder().AddFallback(new FallbackStrategyOptions
{
    Handler = new FallbackHandler()
        .SetFallback<int>(handler =>
        {
            handler
                .ShouldHandle
                .HandleResult(-1)
                .HandleException<InvalidOperationException>();

            handler.FallbackAction = (outcome, context) => new ValueTask<int>(0);
        })
        .SetFallback<double>(handler =>
        {
            handler
                .ShouldHandle
                .HandleResult(-1)
                .HandleException<InvalidOperationException>();

            handler.FallbackAction = (outcome, context) => new ValueTask<double>(0);
        }),
    OnFallback = new OutcomeEvent<OnFallbackArguments>().Register(() => Console.WriteLine("Fallback"))
});

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 #1158 (1dfc954) into main (a1ef1bb) will increase coverage by 0.34%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##             main    #1158      +/-   ##
==========================================
+ Coverage   81.50%   81.85%   +0.34%     
==========================================
  Files         243      253      +10     
  Lines        5673     5780     +107     
  Branches      952      961       +9     
==========================================
+ Hits         4624     4731     +107     
  Misses        844      844              
  Partials      205      205              

Flag	Coverage Δ
linux	?
macos	81.85% <100.00%> (+0.34%)	⬆️
windows	81.85% <100.00%> (?)

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

Impacted Files	Coverage Δ
src/Polly.Core/Fallback/FallbackHandler.Handler.cs	100.00% <100.00%> (ø)
src/Polly.Core/Fallback/FallbackHandler.TResult.cs	100.00% <100.00%> (ø)
src/Polly.Core/Fallback/FallbackHandler.cs	100.00% <100.00%> (ø)
.../Polly.Core/Fallback/FallbackResilienceStrategy.cs	100.00% <100.00%> (ø)
...ack/FallbackResilienceStrategyBuilderExtensions.cs	100.00% <100.00%> (ø)
...y.Core/Fallback/FallbackStrategyOptions.TResult.cs	100.00% <100.00%> (ø)
src/Polly.Core/Fallback/FallbackStrategyOptions.cs	100.00% <100.00%> (ø)
src/Polly.Core/Fallback/HandleFallbackArguments.cs	100.00% <100.00%> (ø)
src/Polly.Core/Fallback/OnFallbackArguments.cs	100.00% <100.00%> (ø)
src/Polly.Core/Fallback/VoidFallbackHandler.cs	100.00% <100.00%> (ø)

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