Dont add to ssh-agent unless specified

还没有人认领这个 Issue。

I would suggest implementing a parameter that allows stopping bless from adding the Identity file to `ssh-agent`.
Namely this lines: https://github.com/lyft/python-blessclient/blob/master/blessclient/client.py#L594-L597

Given a config like:
```
Match exec "env | grep -q BLESS_COMPLETE || /Users/stype/blessclient/blessclient.run --gui --host '%h'"
IdentityFile ~/.ssh/blessid
```
this is not required and could interfere with other credentials.

I think this is required when doing a wrapper around the `ssh` command but in that case its easy to toggle on in the script.

If this is accepted I can create a PR to cleanup/implement this.

从 blessclient/client.py 的第 594-597 行开始，跟踪客户端选项如何传递给 SSH-agent 处理逻辑。添加一个用于禁用添加 Identity 文件的参数，同时默认保留现有行为；当 wrapper 用例可以选择不添加该文件且不影响其他凭据时，即表示完成。

由索引模型根据 Issue 内容生成。