Change interpreters list into an Observable list

[DonJayamanne]

See #2853

Today we need the following from the list of interpreters:

• Do we have any items in the list? (we wait for everything to complete)
• Filtering the list to check if a certain type exists
• Monitor new items in the list
• Standard list operations

Each of the above operations have corresponding operations in an Observable list (any, filter, subsribe, etc - all in non-blocking way).
We need to start using Observable lists, i.e. RxJs (as we're slowly but steadily adding code that comes out of the box with RxJs).

Benefits:

• Simpler api
• Less code
• Faster (non-blocking - as we do not need to wait for everything to complete)

[luabud]

We're changing the way we handle the interpreters list with #12020. So closing as stale.

[DonJayamanne]

@karthiknadig this is my original proposal