feat: Retry failed Event requests

[darielkremov]

Added Polly to retry failed Event requests, as recommended in .NET docs. As the various client SDKs differ in configuration I've set up a minimal config that can be easily adjusted and extended to more APIs (for example flags, config, etc) if needed. The default settings try to match js/python client configs. We cover batch and capture endpoints for now, but not any of the flags or config endpoints to stay consistent with other clients.
We can easily adjust the exceptions or ApiResult statuses that are being retried if needed, for now it retries ApiException and timeouts, other exceptions throw directly.

A ResiliencePipeline can be passed to new PostHogClient(), if null it will create pipeline with default settings from PostHogOptions, or if no retry needed - ResiliencePipeline.Empty executes the callbacks directly with no extra logic.

Honestly, the added test is a bit more of an integration test than unit, but at least helps verify the behavior and executes quickly.

Closes #7

[greptile-apps]

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[github-actions]

Hey, this PR has been open for a while. Someone should look at it, or we can just close it if it's no longer relevant