feat!: improve gherkin files

[aepfli]

I looked at our Gherkin files, and I feel we can optimize them slightly. They're sometimes hard to use.

• Transferred a lot of When into Given
• split flag actions into smaller chunks (one for reason, one for value, one for variant) instead of merging them into one
• added tags throughout the test suite to easier enable disable features (not by loading files, but by defining tags)

⚠️ Breaking Change

• changed how flags etc, are evaluated; current test suites won't work anymore
• Flag evaluation now differs from the spec gherkin file. We could also adapt the spec gherkin file or create one here; I am open to both solutions.

[aepfli]

relates to #166

[aepfli]

open-feature/java-sdk-contrib#1115 and open-feature/python-sdk-contrib#121 and open-feature/js-sdk-contrib#1129

Those need to have a little bit more work still, as they're not 100% feature complete, but show the process in general

[aepfli]

@toddbaert @beeme1mr I would love to move forward with this. I have implemented reference implementations within Java and Python (also using tags to exclude features), which work great (except for Python 3.8, which is a little sad, but I can't fix the tooling).

(teaser: I also have a socket path implementation for Python with this test suite ready)

Let me know what you think :)