[OpAMP][E2E Test] Verify that contrib OTel Collectors can talk to Fleet over OpAMP

[ycombinator]

In #6270, we introduced a /v1/opamp endpoint in Fleet Server. OTel Collectors, specifically a contrib release, We need to write an E2E test in Fleet Server that verifies that a contrib OTel Collector can successfully connect to Fleet Server's OpAMP endpoint.