[flagd] migrate e2e tests to use test containers for container image starts

[aepfli]

Within the Java, Python, and JS SDK, we have already moved to testcontainers.org for testing with the test-harness. It reduces the pain and effort, as the containers are started as part of the tests, rather than an extra ci step and/or locally an additional docker command.

Within our ci files:

dotnet-sdk-contrib/.github/workflows/ci.yml

Lines 50 to 60 in 2f4907e

	services:
	# flagd-testbed for flagd RPC provider e2e tests
	flagd:
	image: ghcr.io/open-feature/flagd-testbed:v0.5.6
	ports:
	- 8013:8013
	# sync-testbed for flagd in-process provider e2e tests
	sync:
	image: ghcr.io/open-feature/sync-testbed:v0.5.6
	ports:
	- 9090:9090

for running the tests locally:
https://github.com/open-feature/dotnet-sdk-contrib/blob/main/src/OpenFeature.Contrib.Providers.Flagd/docker-compose.yaml

See: https://dotnet.testcontainers.org/ how to integrate testcontainers

This issue blocks #306