feat: Add mechanism to ANR event

[bitsandfoxes]

Addresses the improvement part of #1954

In line with Java and the V1 ANR watchdog (documented here), the SDK will now add the ANR mechanism when capturing an ANR event.

Relates to:

• ANR detection - Capture any type of app being killed #1961

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## Unreleased

- Add mechanism to ANR event ([#1955](https://github.com/getsentry/sentry-unity/pull/1955))

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[bruno-garcia]

Missing tests

[bruno-garcia]

nice! Thanks Stefan