add contexts to scope and apply it

[rxlabz]

📢 Type of change

• Bugfix
• New feature
• Enhancement
• Refactoring

📜 Description

add a contexts property to Scope

💡 Motivation and Context

https://develop.sentry.dev/sdk/unified-api/

💚 How did you test it?

📝 Checklist

• I reviewed submitted code
• I added tests to verify changes
• All tests passing
• No breaking changes

🔮 Next steps

[marandaneto]

this PR is not addressing the SentryClient changes:
which is merging the Event Contexts and the Scope Contexts
https://github.com/getsentry/sentry-java/blob/5ebd49cbfd17eb66175e9247db6d27e4df84a56b/sentry/src/main/java/io/sentry/SentryClient.java#L317-L321

[rxlabz]

this PR is not addressing the SentryClient changes:
which is merging the Event Contexts and the Scope Contexts

@marandaneto is this incorrect or incomplete?

sentry-dart/dart/lib/src/scope.dart

Lines 162 to 166 in 262305a

	contexts.clone().forEach((key, value) {
	if (!event.contexts.containsKey(key) && value != null) {
	event.contexts[key] = value;
	}
	});

[marandaneto]

this PR is not addressing the SentryClient changes:
which is merging the Event Contexts and the Scope Contexts

@marandaneto is this incorrect or incomplete?

sentry-dart/dart/lib/src/scope.dart

Lines 162 to 166 in 262305a

	contexts.clone().forEach((key, value) {
	if (!event.contexts.containsKey(key) && value != null) {
	event.contexts[key] = value;
	}
	});

more to incomplete than incorrect, I've made a few new comments

[marandaneto]

LGTM