feat: add http basic auth for receive

[sergeylanzman]

• I added CHANGELOG entry for this change.

Changes

• Added two new flags: --receive.basic-auth-username and --receive.basic-auth-password to configure HTTP Basic Auth for the /api/v1/receive endpoint.
• Implemented middleware that checks the provided username and password against the configured values before processing incoming write requests.
• When both the username and password are set, Basic Auth is enforced. If not set, the endpoint remains open as before.
• Added logging statements to indicate whether Basic Auth is enabled or disabled at startup.

Verification

• Manually tested by running Thanos Receiver locally with different configurations:
  • Without setting username/password, verified that requests are accepted without authentication.
  • With username/password set, verified that requests fail with 401 Unauthorized if credentials are not provided.
  • With valid credentials, verified that requests are processed successfully.
• No regressions were observed in existing functionality.

[MichaHoffmann]

I don't think this should go into thanos itself. Maybe some system build on top of thanos can enforce authentication.