Allow implicit conversion of PredicateBuilder to delegates

[martintmk]

Details on the issue fix or feature implementation

Contributes to #1329

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• 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 #1332 (6faa97d) into main (73422d5) will decrease coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1332      +/-   ##
==========================================
- Coverage   83.53%   83.49%   -0.05%     
==========================================
  Files         269      270       +1     
  Lines        6397     6379      -18     
  Branches     1003     1003              
==========================================
- Hits         5344     5326      -18     
  Misses        844      844              
  Partials      209      209              

Flag	Coverage Δ
linux	83.49% <100.00%> (?)
macos	83.49% <100.00%> (-0.05%)	⬇️
windows	83.49% <100.00%> (-0.05%)	⬇️

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Impacted Files	Coverage Δ
...ack/FallbackResilienceStrategyBuilderExtensions.cs	100.00% <ø> (ø)
.../Retry/RetryResilienceStrategyBuilderExtensions.cs	100.00% <ø> (ø)
src/Polly.Core/PredicateBuilder.Operators.cs	100.00% <100.00%> (ø)
src/Polly.Core/PredicateBuilder.TResult.cs	100.00% <100.00%> (ø)