This adds support for SSH-style Git URLs, enabling folks to use
private repos for their templates.
For instance,
$ pulumi new git@github.com:acmecorp/templates/website
will now work as intended.
The `ssh_config` library handles finding the relevant SSH key for the
given host.
If the SSH key is protected by a password, the user will be prompted to
supply the password on-demand. (It is memoized to avoid asking multiple
times, as the template workflow requires using it more than once.) To
avoid prompting, the `PULUMI_GITSSH_PASSPHRASE` env var can be set.
Fixes#4872 and #5007.
