Support SSH usernames which contain @ symbols (#25314)

Closes #25246

Release Notes:

- SSH: Improved handling of multiple `@` in connection strings: e.g.
`ssh jim.lv@es2@10.220.67.57@11.239.1.231` improving support of jump
hosts running JumpServer.

---------

Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>