In-url HTTP basic auth for the test commands

[OisinKyne]

🎯 Problem to be solved

We have a new auth protected bn I want to load test. I want to use the charon alpha test beacon command to do it. I can't get past the auth.

🛠️ Proposed solution

• Approved design doc: link
• Core team consensus on the proposed solution

We could add a flag for --endpoint-headers or something, but keeping it simpler, maybe a https://user:pass@domain.com format is easiest?

🧪 Tests

• Tested by new automated unit/integration/smoke tests
• Manually tested on core team/canary/test clusters
• Manually tested on local compose simnet