Support configuration arguments via `--sentry-define`

[denrase]

:scroll: Description

Tried using Dart Environment Declarations, but failed for several reasons:

• Can't run the plugin using dart run in a flutter app (integration test app)

denis@Deniss-MBP integration-test % dart run sentry_dart_plugin
The Flutter SDK is not available.

• Values not read, only when specifying a dart source file 'dart run lib/main.dart'. Because of that we cannot use it in our plugin.

It seems plugins can use the arguments users pass, so I opted to implement our own declarations using a similar syntax:

--sentry-define=<KEY>=<VALUE>

With which users can pass any of our supported keys.

flutter packages pub run sentry_dart_plugin --sentry-define=release=app-internal-test@0.0.1

:bulb: Motivation and Context

Closes #81

:green_heart: How did you test it?

• Unit tests
• Updated integration test

:pencil: Checklist

• [x] I reviewed submitted code
• [ ] I added tests to verify changes
• [ ] I updated the docs if needed
• [x] All tests passing
• [x] No breaking changes

[github-actions[bot]]

	Messages
:book:	Do not forget to update Sentry-docs with your feature once the pull request gets approved.

Generated by :no_entry_sign: dangerJS against eee9ceb630810ad067bb4ebc1e34a9bb07694e6c

[eb-so]

Any new updates on this feature?

[denrase]

@eb-so Sorry for the delay. I updated the PR and now it's ready for review.

[eb-so]

@eb-so Sorry for the delay. I updated the PR and now it's ready for review.

No worries, thanks for the update

[denrase]

@stefanosiano Could you take a look here? 👀

[denrase]

@buenaflor Could you re-review here to check if everything is as expected?

[denrase]

@buenaflor Let's merge the other PR and then update here.

[buenaflor]

Needs updating of the comments as soon as https://github.com/getsentry/sentry-dart-plugin/pull/216 is merged