fix: HTTP transport on Mono with high stripping

[bitsandfoxes]

Surfaced by #2854. That's the first time the Windows Mono job ran on Unity 6, where we build with High stripping.

On Mono, HttpWebRequest spins up the configuration system, which creates ExeConfigurationHost and the machine.config section types via reflection. The linker can't see that and strips them. So every envelope fails to send with Default constructor not found for type System.Configuration.ExeConfigurationHost. This isn't Sentry-specific, but it takes out our transport.

Unity ignores link.xml files inside packages, and we stopped writing one into Assets/ back in #707. So we're now providing the rules at build time via IUnityLinkerProcessor, and only for Mono.

The DependencyConflict package also breaks on Mono + High stripping (FieldAccessException in its own unaliased System.Text.Json). That's not ours to fix and Sentry doesn't go through that path, so it's skipped on the Mono job for now.

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- HTTP transport on `Mono` with high stripping ([#2858](https://github.com/getsentry/sentry-unity/pull/2858))

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