Update env/config `release` and `dist` behaviour

[denrase]

:scroll: Description

Default Release

• Per default, the release is build from pubspec.yaml's name & version: name@version
• If a build number is present in version, it is used as dist

Providing custom release from env. variable or plugin config

• It is used as release and is not mutated any further

Providing custom dist from env. variable or plugin config

• It's used as dist and is not mutated any further
• It's added as or replacing the build number in default release

Env Variables

• SENTRY_RELEASE and SENTRY_DIST have precedence over values from plugin config

:bulb: Motivation and Context

Closes #207

:green_heart: How did you test it?

Unit tests

:pencil: Checklist

• [x] I reviewed submitted code
• [x] I added tests to verify changes
• [ ] I updated the docs if needed
• [x] All tests passing
• [ ] 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 9e311663127eefd3aa20bb8640fa2b01ae1f2ec0

[denrase]

@stefanosiano Please add your 2 cents here, as this is changing current behaviour. 🙇

[buenaflor]

Since this is breaking, should we consider releasing it as 2.0.0 ?

[denrase]

@buenaflor Added info in readme. This PR points to #216. So when we merge here we can check for 2.0 release. WDYT?

[buenaflor]

@denrase sounds good