Implement Variants API

[kmasiello]

See https://pkgs.rstudio.com/connectapi/reference/VariantR6.html

Notably, the workflow we would like to enable with the sdk is triggering a variant to re-render and then subsequently send an email. Something along the lines of:

client = connect.Client()
content_variant = client.varient.find("default")
new_render =  content_variant.render()
new_render.send_mail(to="me")

Also see rstudio/connectapi#257

[tdstein]

Partially implemented in #236