tool: add a new email tool and an example agent for sending email

[201430098137]

add email sending tool feature
1.add email tool
2.add example to use email tool

[github-actions]

CLA Assistant Lite bot All contributors have signed the CLA ✍️ ✅

[201430098137]

I have read the CLA Document and I hereby sign the CLA

[codecov]

Codecov Report

❌ Patch coverage is 87.14286% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.62805%. Comparing base (16307d7) to head (87c608f).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
tool/email/sendmail.go	85.45455%	10 Missing and 6 partials ⚠️
tool/email/email.go	93.33333%	2 Missing ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main        #714         +/-   ##
===================================================
- Coverage   87.63538%   87.62805%   -0.00733%     
===================================================
  Files            281         283          +2     
  Lines          36564       36704        +140     
===================================================
+ Hits           32043       32163        +120     
- Misses          2937        2950         +13     
- Partials        1584        1591          +7     

Flag	Coverage Δ
unittests	87.62805% <87.14286%> (-0.00733%)	⬇️

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