fix: add back AuthBackend which is required by custom auth backend

[sunng87]

I hereby agree to the terms of the GreptimeDB CLA.

Refer to a related PR or issue link (optional)

#4359

What's changed and what's your intention?

AuthBackend error is designed to represent errors from authentication data source. It's deleted in #4359 . This patch brings it back and updated its message

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR requires documentation updates.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.76%. Comparing base (2ae2a66) to head (2940e03).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4409      +/-   ##
==========================================
- Coverage   85.03%   84.76%   -0.28%     
==========================================
  Files        1075     1075              
  Lines      192553   192553              
==========================================
- Hits       163743   163210     -533     
- Misses      28810    29343     +533     

[fengjiachun]

LGTM

[shuiyisong]

Rest LGTM