Move the LegacyFolder outside of Polly.Core

[martintmk]

The issue or feature being addressed

In this PR we move the LegacyFolder outside of Polly.Core so it can be reused by other projects. Additionaly we add support for:

• Records
• Full set of nullability attributes

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 #1083 (39ea89e) into main (7688615) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##             main    #1083      +/-   ##
==========================================
- Coverage   73.83%   73.80%   -0.04%     
==========================================
  Files         161      161              
  Lines        4086     4081       -5     
  Branches      775      775              
==========================================
- Hits         3017     3012       -5     
  Misses        864      864              
  Partials      205      205              

Flag	Coverage Δ
linux	73.80% <100.00%> (-0.04%)	⬇️
macos	73.80% <100.00%> (-0.04%)	⬇️
windows	73.80% <100.00%> (-0.04%)	⬇️

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Impacted Files	Coverage Δ
...rc/Polly.Core/Builder/ResilienceStrategyBuilder.cs	100.00% <100.00%> (ø)

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