Support custom units for custom measurements

[marandaneto]

Description

Due to the limitations of enums on Dart, there was no easy way to extend the enum and accept a plain String.
We can leverage this feature if needed, when we upgrade to Dart 2.17, or just set the unit as a plain string and document all the possible values.

Related to #1011

Example on Java:

transaction.setMeasurement("metric1", 2, MeasurementUnit.Duration.DAY)
transaction.setMeasurement("metric1", 2, MeasurementUnit.Custom("myOwnUnit"))

[marandaneto]

On v7, we can do this:

environment:
  sdk: '>=2.17.0 <3.0.0'
  flutter: '>=3.0.0'

So we can use new features and unblock this, Flutter people upgrade fast.

[marandaneto]

Example: https://github.com/getsentry/sentry-java/blob/e2433098d0addc8570750857f9f20c33ab28143c/sentry/src/main/java/io/sentry/MeasurementUnit.java

[denrase]

@marandaneto I'm not sure how the new enum features of the Language are of any help here. While enum cases can take associated values, they have to be declared with the enum case, and cannot be instantiated freely with any value. So this is different than enums in Swift or sealed classes in Kotlin. Am I missing something here?

https://dart.dev/guides/language/language-tour#declaring-enhanced-enums