Setting env variables via deploy_repo

[fh-mthomson]

Motivating context: we'd like users to be able to deploy to Connect BOTH:

1. via Git (connectapi: already supported; rsconnect: see Deploy via Git using rsconnect rstudio/rsconnect#888), AND:
2. Setting env vars at the time of deployment (connectapi: newly-supported, rsconnect: this issue)

For connectapi, I'm envisioning a parallel version of rstudio/rsconnect#859 (comment):

Sys.setenv(SECRET = "forty-two")

connectapi::deploy_repo(
    client = connectapi::connect(),
    repository = "X",
    branch = "Y",
    subdirectory = "Z",
    envVars = c("SECRET")
 )