[Enhancement]Update system message

[shiyutang]

• 增加system message 在memory.get_message中展示。
• 同步agent中设置的system message到memory。
• 后续不再通过memory设置system message，只保留agent中同步入口。

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (6fbc099) 69.37% compared to head (c0c5446) 69.64%.

Files	Patch %	Lines
erniebot-agent/src/erniebot_agent/agents/agent.py	91.66%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #272      +/-   ##
===========================================
+ Coverage    69.37%   69.64%   +0.26%     
===========================================
  Files           63       63              
  Lines         3236     3245       +9     
===========================================
+ Hits          2245     2260      +15     
+ Misses         991      985       -6     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[Southpika]

诗雨再拉一次271～给functionagent_with_retrieval 加了一个单测，其他没有意见～

[Bobholamovic]

可能还需要修改下mock_memory的实现～

[shiyutang]

done~