chore: flagd gherkin improvements

[aepfli]

This PR

• using gherkin auto binding as described in #986
• unify the location for the test harness, spec and the auto binding files within flagd-core instead of flagd and flagd-web
• removing file operations to move the feature files before execution into a seperate folder (no need for this anymore)
• removed step definitions and constants of e2e tests from the built package

Related Issues

Fixes #986

Follow-up Tasks

currently the path for the gherkin feature files is 'hardcoded' and not autmatically resolved relatively to the test (maybe somebody has a good idea for this)

[aepfli]

okay, i failed here hard, i did not check the api, and the webclient and the server client do not have an overlapping api - hence that part of this improvements are not worth it. eg. the normalization witthin flagd-core. I could generate an wrapper around it, to be passed on to the stepdefinitions, but imho i do not think that this is worth it.