Enhance stream tap creation experience

[ilayaperumalg]

As a user I'd like to have my stream application's endpoints visible at the stream creation page so that I could create tap on the streams that were created already.

For instance, let's say I have a stream: s1=http | transform | log already created.

When I try to create a new stream, I can have the named destinations such as s1.http and s1.transform to be available so that I could choose those for the new tap streams I newly create.

It would also be better to show the original stream graph (in this case s1) and load their named destinations as well.