Test jaeger example and align examples with docs

[pichlermarc]

This PR

• aligns the examples with our documentation by
  • using DT_ENDPOINT as the env var as opposed to DT_OTLP_ENDPOINT
  • using grpc as the default for the jaeger reciever instead of thrift_http
• adds a test for the Jaeger example using the collector testbed
  • adds a dependency to github.com/open-telemetry/opentelemetry-collector-contrib/testbed/datasenders so that we can use the existing Jaeger gRPC data sender.

depends-on: #132 (removing examples that were meant for a different audience)