Introduce a way to capture libssh logs

**What are you trying to do?**
libssh emits its own logs, which could be very useful for debugging purposes. As of now, we completely ignore them. There are the following levels:
| Level | Description |
|-|-|
| `SSH_LOG_NOLOG` | No logging at all |
| `SSH_LOG_WARNING` | Only warnings |
| `SSH_LOG_PROTOCOL` | High level protocol information |
| `SSH_LOG_PACKET` | Lower level protocol infomations, packet level |
| `SSH_LOG_FUNCTIONS` | Every function path |

I think the most useful one for debugging would be `SSH_LOG_PROTOCOL`.

**What's your proposed solution?**
There could be several approaches:
1. Log everything emitted by libssh under our own `trace` level. We could always set the libssh verbosity level to `SSH_LOG_PROTOCOL`, capture every log and log it using the Multipass logging facilities, under the `trace` level.
2. Read an environment variable (perhaps `MULTIPASS_LIBSSH_VERBOSITY`) for the value we want to pass to `ssh_set_log_level`.
3. Combine approach 1 and 2. Capture everything from `SSH_LOG_PROTOCOL` and log under `trace`, but only if an environment variable is set (perhaps `MULTIPASS_LIBSSH_LOGGING`).

**Additional context**
The need for this comes from #3552.

Start by reviewing the libssh logging levels and its ssh_set_log_level API, then inspect the Multipass logging facilities. Resolve which proposed capture and environment-variable approach is wanted; done means useful libssh logs are captured through Multipass logging at the intended verbosity without changing normal behavior.

Written by the indexing model from the issue text.