ResilienceStrategyRegistry API improvements

[martintmk]

Details on the issue fix or feature implementation

Bunch of API improvements for working with ResilienceStrategyRegistry

• Adding GetOrAddStrategy method for non-generic and generic strategies.
• Adding AddResilienceStrategy<TKey> extension that adds all the necessary infra without requiring to configuring any strategy.
• Allowing to enable reloads when using the registry directly.

Api Usage:

var registry = new ResilienceStrategyRegistry<string>();
IOptionsMonitor<MyOptions> monitor = ... ;

ResilienceStrategy strategy = registry.GetOrAddStrategy("my-strategy", (builder, context) =>
{
    context.EnableReloads(monitor);
    builder.AddTimeout(monitor.CurrentValue.Timeout);
});

Contributes to #1365

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 #1388 (b936db2) into main (20598ad) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1388      +/-   ##
==========================================
+ Coverage   83.68%   83.72%   +0.03%     
==========================================
  Files         269      270       +1     
  Lines        6431     6445      +14     
  Branches     1004     1005       +1     
==========================================
+ Hits         5382     5396      +14     
  Misses        840      840              
  Partials      209      209              

Flag	Coverage Δ
linux	?
macos	83.72% <100.00%> (+0.03%)	⬆️
windows	83.72% <100.00%> (+0.03%)	⬆️

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

Impacted Files	Coverage Δ
...ore/Registry/ResilienceStrategyRegistry.TResult.cs	100.00% <100.00%> (ø)
.../Polly.Core/Registry/ResilienceStrategyRegistry.cs	100.00% <100.00%> (ø)
...ependencyInjection/AddResilienceStrategyContext.cs	100.00% <100.00%> (ø)
...dencyInjection/PollyServiceCollectionExtensions.cs	100.00% <100.00%> (ø)
...ions/Registry/ConfigureBuilderContextExtensions.cs	100.00% <100.00%> (ø)