feat(auth): watch file user provider

[tisonkun]

I hereby agree to the terms of the GreptimeDB CLA.

Refer to a related PR or issue link (optional)

What's changed and what's your intention?

This closes #1862.

Implement greptime standalone start --user-provider=watch_file_user_provider:./users.toml that would watch on the file for updates.

This is what @thinkORo wants in the original issue, but in a way more suitable for our model.

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR does not require documentation updates.

[codecov]

Codecov Report

Attention: Patch coverage is 78.08219% with 48 lines in your changes are missing coverage. Please review.

Project coverage is 84.85%. Comparing base (dd18d8c) to head (67219e6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3566      +/-   ##
==========================================
- Coverage   85.21%   84.85%   -0.37%     
==========================================
  Files         924      925       +1     
  Lines      153524   153676     +152     
==========================================
- Hits       130831   130401     -430     
- Misses      22693    23275     +582     

[killme2008]

Good job! Looks cool.

[shuiyisong]

Please resolve the comments. Let's go 🚀

[tisonkun]

All addressed. I may wait a bit for any feedback on #3566 (comment); otherwise, I'll merge this one tomorrow.

[tisonkun]

I'll merge this as is now.

If you have other ideas on #3566 (comment), please drop a comment. We still have the chance to make changes before 0.8 or even 1.0 release.

[killme2008]

@tisonkun Looks like we need to update the doc https://docs.greptime.com/user-guide/clients/authentication#authentication

[tisonkun]

Thanks for pointing out the doc source. I'll update for this and the admin /maintenance endpoint in days.