Introduce ResilienceEventSeverity

[martintmk]

Details on the issue fix or feature implementation

Even though we are feature complete I want to squeeze this in to drop some workarounds around ExecutionAttempt event that should have informational severity. This change allows that.

Closes #1143

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 #1361 (4587a7a) into main (190f096) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1361      +/-   ##
==========================================
+ Coverage   83.56%   83.61%   +0.05%     
==========================================
  Files         268      268              
  Lines        6382     6403      +21     
  Branches     1001     1004       +3     
==========================================
+ Hits         5333     5354      +21     
  Misses        840      840              
  Partials      209      209              

Flag	Coverage Δ
macos	83.61% <100.00%> (+0.05%)	⬆️
windows	83.61% <100.00%> (+0.05%)	⬆️

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

Impacted Files	Coverage Δ
...ircuitBreaker/Controller/CircuitStateController.cs	100.00% <100.00%> (ø)
.../Polly.Core/Fallback/FallbackResilienceStrategy.cs	100.00% <100.00%> (ø)
...rc/Polly.Core/Hedging/HedgingResilienceStrategy.cs	100.00% <100.00%> (ø)
src/Polly.Core/ResilienceContext.cs	100.00% <100.00%> (ø)
src/Polly.Core/Retry/RetryResilienceStrategy.cs	100.00% <100.00%> (ø)
src/Polly.Core/Telemetry/ResilienceEvent.cs	100.00% <100.00%> (ø)
...olly.Core/Telemetry/ResilienceStrategyTelemetry.cs	100.00% <100.00%> (ø)
...lly.Core/Telemetry/TelemetryEventArguments.Pool.cs	100.00% <100.00%> (ø)
...rc/Polly.Core/Telemetry/TelemetryEventArguments.cs	100.00% <100.00%> (ø)
src/Polly.Core/Telemetry/TelemetryUtil.cs	100.00% <100.00%> (ø)
... and 6 more