Add function to get an integration template

This is needed so that users of create_integration() and update_integration() can know the valid values for the arguments to those functions. (See here.)

Since templates aren't passed around as objects, and are more just there for reference, it's reasonable to not worry too much about class stuff with this.

client <- connect()
templates <- get_templates(client)

I don't know that the "get a single template" endpoint is useful to implement, since it requires the template ID. In Integration objects, templates are specified by name, so to get the ID you'd already have had to get_templates() and filtered by name to get the ID.