ssh: support multiple keys

[isidentical]

Resolves #6508

[Erotemic]

Should there ( / could there) be a DEBUG logging step here?

# In dvc/fs/ssh.py
import rich
import logging
logger = logging.getLogger(__name__)

...
       # <in: SSHFileSystem._prepare_credentials>

        login_info_display = login_info.copy()
        if 'password' in login_info_display:
            login_info_display['password'] = '<redacted>'
        logger.debug(
            "login_info = {}".format(rich.pretty.pretty_repr(login_info_display))
        )

I can make the PR if this change if adding ssh info to the debug output is desirable. (It would make me feel better as a sanity check if I could see it in the future).

[isidentical]

I don't think this should be done, not at least on the SSH level. The function is called _prepare_credentials since it might contain any sort of confidential data (e.g the host IP, the username, or the password as you simply redacted) and generally we ask users to submit their whole debug log to us through discord, which is a public place so people might be even unaware that they are sharing such info, and some might even not to do so. So I'd say that since DVC is a python program, it is much simpler to just insert a breakpoint() and restart.