[flagd] e2e test for flagd and flagd-web

[toddbaert]

This should be similar to what's in the js-sdk now.

Definition of done:

• run e2e tests based on test-harness ghekin suite and flagd test image. A lot of the code can be taken from the client and server implementations in teh js-sdk.
• run in CI

[aepfli]

@toddbaert I think this is now also resolved, or am I missing something?