Simplify handling of reloads

[martintmk]

Details on the issue fix or feature implementation

Dynamic reloads implementation can be simplified with a simple change. Instead of passing the Func<CancellationToken> we actually use a factory that produces the Func<CancellationToken>. This allows us to activate the factory only once when the strategy is being created and we do not have to handle multiple occurrences of Func<CancellationToken>.

As a part of this PR I am also hiding some API which might not be used outside of Polly. We can still unhide later.

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]

Codecov Report

Merging #1374 (c2bdb7a) into main (70129e2) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1374      +/-   ##
==========================================
- Coverage   83.61%   83.57%   -0.04%     
==========================================
  Files         268      267       -1     
  Lines        6403     6388      -15     
  Branches     1004     1002       -2     
==========================================
- Hits         5354     5339      -15     
  Misses        840      840              
  Partials      209      209              

Flag	Coverage Δ
macos	83.57% <100.00%> (-0.04%)	⬇️
windows	83.57% <100.00%> (-0.04%)	⬇️

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Impacted Files	Coverage Δ
...dencyInjection/PollyServiceCollectionExtensions.cs	100.00% <ø> (ø)
src/Polly.Core/Registry/ConfigureBuilderContext.cs	100.00% <100.00%> (ø)
.../Polly.Core/Registry/ResilienceStrategyRegistry.cs	100.00% <100.00%> (ø)
...ependencyInjection/AddResilienceStrategyContext.cs	100.00% <100.00%> (ø)
src/Polly.Extensions/Utils/OptionsReloadHelper.cs	100.00% <100.00%> (ø)