rpcclient: allow disabling basic auth

[0xjc65eth]

Summary

• Add ConnConfig.DisableAuth so clients can skip HTTP Basic Auth.
• Do not call getAuth or send Authorization when disabled, including HTTP POST mode and websocket dialing.
• Add regression coverage that default POST requests still send Basic Auth, while disabled-auth POST requests work without credentials or an Authorization header.

Fixes #2505.

Tests

• go test ./rpcclient -run 'TestSendPostRequestWithRetry(UsesBasicAuthByDefault|DisableAuth|Success)$' -count=1
• go test ./rpcclient -count=1
• git diff --check

[TechLateef]

Thanks for opening this! Just a heads-up that this issue has already been handled and approved over in PR #2514 here. You might want to take a look at that thread!