feat: 添加用户邮箱

[he0119]

方便向用户发送通知或者其他需要邮箱的地方。

[codecov]

Codecov Report

❌ Patch coverage is 96.29630% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 97.02%. Comparing base (87a92c3) to head (b0d5668).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
nonebot_plugin_user/utils.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #184      +/-   ##
==========================================
- Coverage   97.18%   97.02%   -0.17%     
==========================================
  Files           7        7              
  Lines         213      235      +22     
==========================================
+ Hits          207      228      +21     
- Misses          6        7       +1     

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