Enable dynamic reloads

[martintmk]

The issue or feature being addressed

Closes #1241

Details on the issue fix or feature implementation

This PR adds opt-in support for reloadable pipelines. Dynamic reloads are enabled when new ConfigureBuilderContext.EnableReloads method is called.

Caller needs to provide a factory that returns a CancellationToken. When the token is cancelled the strategy is reloaded.

I have also considered using IChangeToken which is more natural to propagate changes, however, we would have to add a new dependency to Microsoft.Extensions.Primitives which seemed overkill for such small addition.

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 #1254 (752bc14) into main (70d1bd9) will increase coverage by 0.16%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1254      +/-   ##
==========================================
+ Coverage   83.12%   83.28%   +0.16%     
==========================================
  Files         269      270       +1     
  Lines        6239     6299      +60     
  Branches      976      978       +2     
==========================================
+ Hits         5186     5246      +60     
  Misses        844      844              
  Partials      209      209              

Flag	Coverage Δ
macos	83.28% <100.00%> (+0.16%)	⬆️
windows	83.28% <100.00%> (?)

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

Impacted Files	Coverage Δ
src/Polly.Core/Registry/ConfigureBuilderContext.cs	100.00% <100.00%> (ø)
.../Polly.Core/Registry/ResilienceStrategyRegistry.cs	100.00% <100.00%> (ø)
...c/Polly.Core/Utils/ReloadableResilienceStrategy.cs	100.00% <100.00%> (ø)

[martintmk]

@martincostello , anything left to address here?