Add support for deploy keys in install_github

[rvernica]

Is your feature request related to a problem? Please describe.

To use install_github with a private repository a Personal Access Token (PAT) is required. This is not ideal for teams as PATs are linked to an individual.

Describe the solution you'd like

Add support for deploy keys in install_github. A deploy key is linked to a repository and not an individual.

Describe alternatives you've considered

An alternative would be to use the deploy key and checkout the private repository. Then install the R package from the local directory.

[jennybc]

devtools::install_github() is actually remotes::install_github(), so I'm transferring this issue.

[gaborcsardi]

Can you use install_git() instead? That should work with ssh keys I believe.

install_github() uses the GitHub API, and I am not sure if the API works with ssh keys.