chore: restructure dotnet build targets for local development

[bitsandfoxes]

QOL feature for local development, so we (I) don't have to clean and build with every change.
Sample usage:

• dotnet build - builds the entire SDK stack including native sdks
• dotnet msbuild /t:BuildAndroidSDK src/Sentry.Unity cleans the output artifacts, and rebuilds sentry-native and sentry-java

#skip-changelog

[bitsandfoxes]

These are the targets used to build players for tests, or utility to help with setup.

[bitsandfoxes]

These targets are intended to allow for iterative building, to overwrite the output inside of package-dev/Plugins/ on each call. No more manual deletion before building.
The Ensure targets are hooked up to the regular build to allow calling dotnet build from the root and the repo automatically builds itself and its native layer.

[bitsandfoxes]

This is a helper to build sentry-native for sentry-java locally. These will get picked up from sentry-java via mavenLocal.

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