…st SSH support (#1781)

Closes #1775
Closes #1452
Closes #1514
Follows #1717

libgit2 does not support host names with extra identifiers during SSH
lookup (e.g. [`github.com-some_identifier`](

https://docs.github.com/en/authentication/connecting-to-github-with-ssh/managing-deploy-keys#using-multiple-repositories-on-one-server))
so we use the `git` command instead for fetching. This is required for
`pip` parity.

See the [Cargo
documentation](https://doc.rust-lang.org/nightly/cargo/reference/config.html#netgit-fetch-with-cli)
for more details on using the `git` CLI instead of libgit2. We may want
to try to use libgit2 first in the future, as it is more performant
(#1786).

We now support authentication with:

```
git+ssh://git@<hostname>/...
git+ssh://git@<hostname>-<identifier>/...
```

Tested with a deploy key e.g.

```
cargo run -- \
    pip install uv-private-pypackage@git+ssh://git@github.com-test-uv-private-pypackage/astral-test/uv-private-pypackage.git \
    --reinstall --no-cache -v
```

and

```
cargo run -- \
    pip install uv-private-pypackage@git+ssh://git@github.com/astral-test/uv-private-pypackage.git \
    --reinstall --no-cache -v     
```

with a ssh config like

```
Host github.com
        Hostname github.com
        IdentityFile=/Users/mz/.ssh/id_ed25519

Host github.com-test-uv-private-pypackage
        Hostname github.com
        IdentityFile=/Users/mz/.ssh/id_ed25519
```

It seems quite hard to add test coverage for this to the test suite, as
we'd need to add the SSH key and I don't know how to isolate that from
affecting other developer's machines.