Add optional filter to subscription creation

[femiaf13-precisionOT]

Currently CreateNotificationStream allows you to specify the stream but not the filter. RFC for reference: https://datatracker.ietf.org/doc/html/rfc5277#section-2.1.1

Here's an example I found of creating a subscription with a filter:

<create-subscription xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0">
     <filter netconf:type="xpath" xmlns:ex="http://example.com/event/1.0"
         select="/ex:event[ex:eventClass='fault' and
             (ex:severity='minor' or ex:severity='major'or ex:severity='critical')]"
     />
</create-subscription>