Get more accurate __dir__ on TelstateToStr
Get more accurate __dir__ on TelstateToStr

The `keys` and `prefixes` attributes can be removed from `TelstateToStr`
since they will be retrieved via the custom __getattr__ from the
underlying `TelescopeState` object (and they don't override the default
versions anymore).

In return, improve the output of __dir__ to account for the public
attributes reachable in the underlying telstate object. This restores
sanity when introspecting the object in IPython. Disregard attributes
starting with a single underscore as well to discourage their access,
as these "private" instance attributes don't live on the class and will
get unintentionally passed through `to_str` (i.e. the bytes-based
`_prefixes` will look just like the string-based `prefixes` via this
mechanism).

ska-sa/katdal