Use same default environment

[philipphofmann]

The default value for the environment in the options should be production. If the user sets an environment, it precedes the default value. Sessions, transactions, events, etc., should all read the environment from the options. The SDK can set other default values if it detects different modes, such as debug or test.

On Cocoa, we have a bug where sessions don't use the default environment from the options. Not setting an environment at all leads to different environments for events and transactions. Please validate that your SDK uses the correct default value:

### Tasks
- [x] https://github.com/getsentry/sentry-cocoa/issues/2411
- [ ] https://github.com/getsentry/sentry-java/issues/2381
- [ ] https://github.com/getsentry/sentry-react-native/issues/2801
- [X] Dart
- [ ] .NET

[marandaneto]

Since Android and iOS don't set/read the environment, RN and Flutter sessions won't contain the default/custom environment, this has to be fixed first on the Native SDKs.
Flutter events are already set/read from the options, but sessions are handled by the native SDKs.
I believe RN has the same issue as Flutter, @krystofwoldrich to double check.

[krystofwoldrich]

Events and Transaction have the default production set by the JS SDK,
Sessions are managed by the Native SDKs.

RN doesn't set any default environment by itself, it only passes the set env value to the native SDKs if there is any value.

[marandaneto]

Can RN detect if it's a release or debug build? if so, we can do that, otherwise, it is production by default, something to be addressed then.