feat: Add OptionConfiguration

[bitsandfoxes]

Fixes #1621

Based on the issue description: The aim is to have users no longer need to know or care about Runtime and BuildTime differences. They write their code in once place, we use the same options at any time.

If the need arises to set native-specific options - i.e. to set the sample-rate on the native layer, we provide NativeOverrides on the options themselves.

public override void Configure(SentryUnityOptions options)
{
    // Here you can programmatically modify the Sentry option properties used for the SDK's initialization

    options.Environment = "dev";

    options.NativeOverrides.Environment = "native_dev";
    options.NativeOverrides.SampleRate = 0.1f;
}

The downside (and a possible footgun for us): We need to be careful when modifying or adding options on the native layer to make sure we use the overrides and if those are null, fall back to the actual options.

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## Unreleased

- Add `OptionConfiguration` ([#1888](https://github.com/getsentry/sentry-unity/pull/1888))

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[bruno-garcia]

Is this a real use case though?
Shouldn't we just take the dev above and automatically override native?

I see this makes sense for things that only the native layer has though