Add custom session properties and session identifiers

[dutysloth]

Hi,
It would be very helpful if there was an option to set custom session properties.
In the current state there is only a limited set of built-in properties that can be set.

I want to be able to run something like this:

SET SESSION myvar = 'abcd'

(without pre-defining 'myvar' in a custom connector or elsewhere...)

For example in my use case, I have Hive views with a UDF in them, that expects some session property to be defined and apply logic based on the value.

Also, it would be nice to have a session identifier (UUID) that can be retrieved using SHOW SESSION...