Make `greeting` an active binding (R) / property (Python)

[cpsievert]

As a follow up to #109, see #109 (comment)

@gadenbuie wrote:

I've been thinking about this a bit and I'd like to explore having greeting be set up as an active binding. We'd have a private$greeting that stores the greeting, and then $greeting() can be a getter/setter. If we you call $greeting() and we don't have one stored yet, we call generate_greeting(), store it and then re-use it.

I think this would eliminate the need for has_greeted and instead we can restore the greeting.

I guess this brings up the question of how we'd want to handle greetings, for which there are four options.

1. If not provided, always generate a greeting on every new session.
2. If not provided, generate on the first session and then re-use each session.
3. If not provided, don't greet.
4. If provided, always use that greeting.

Should we support all of these? If so, we could have a generate_greeting = c("once", "always", "never") argument in QueryChat$new().