Created ydb_sup_utils module.

Defines function get_pid/2 that returns the pid associated with the
specified child.
Implemented ydb_sup_utils:pid_fun/1.

Creates a closure for get_pid/2. Returns a function that takes no
arguments and resolves to the pid of the specified child.
Added support for plan node to attach self as listener.

Takes a pid and calls add_listener/2. In the future, add_listener/2
will return the schema of the plan node it was called on instead of
simply returning 'ok' as it does now. This will greatly simplify the
amount of information that needs to be specified in a query.