Introduce TelemetryOptions.OnTelemetryEvent

[martintmk]

Details on the issue fix or feature implementation

This comment:
martincostello/polly-sandbox#1 (comment)

Made me realize that not every adopter wants to consume native Polly telemetry. This PR adds a simple way to hook into Polly telemetry and inspect TelemetryEventArguments that contain all necessary information to report custom events from it.

Usage:

var builder = new ResilienceStrategyBuilder()
    .AddTimeout(TimeSpan.FromSeconds(1))
    .ConfigureTelemetry(new TelemetryOptions
    {
        OnTelemetryEvent = args =>
        {
            // do whatever you want with args
        }
    });

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 #1387 (8cd00cc) into main (ed4c480) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1387   +/-   ##
=======================================
  Coverage   83.68%   83.68%           
=======================================
  Files         269      269           
  Lines        6429     6431    +2     
  Branches     1003     1004    +1     
=======================================
+ Hits         5380     5382    +2     
  Misses        840      840           
  Partials      209      209           

Flag	Coverage Δ
linux	?
macos	83.68% <100.00%> (+<0.01%)	⬆️
windows	83.68% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
.../Polly.Core/Telemetry/ResilienceTelemetrySource.cs	100.00% <ø> (ø)
...rc/Polly.Core/Telemetry/TelemetryEventArguments.cs	100.00% <ø> (ø)
...s/Telemetry/ResilienceTelemetryDiagnosticSource.cs	100.00% <100.00%> (ø)
src/Polly.Extensions/Telemetry/TelemetryOptions.cs	100.00% <100.00%> (ø)