Introduce Outcome.EnsureSuccess

[martintmk]

Details on the issue fix or feature implementation

This PR introduces Outcome.EnsureSuccess API and does some small cleanup.

Contributes to #1238.

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 #1243 (1b80405) into main (4ec50a9) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1243      +/-   ##
==========================================
+ Coverage   82.79%   82.81%   +0.01%     
==========================================
  Files         265      265              
  Lines        6096     6103       +7     
  Branches      966      967       +1     
==========================================
+ Hits         5047     5054       +7     
  Misses        844      844              
  Partials      205      205              

Flag	Coverage Δ
linux	82.81% <100.00%> (?)
windows	?

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)
src/Polly.Core/Strategy/Outcome.cs	100.00% <100.00%> (ø)
...ependencyInjection/AddResilienceStrategyContext.cs	100.00% <100.00%> (ø)