[Canvas] Create generic "embeddable" function

[crob611]

There should be an "embeddable" function that takes in an input argument that is just a big config object. This will allow for by reference or by value embeddables.

• The underlying config should be "hidden" from the user when viewing the expression. Because it will just be a big json blob, the user will not be directly editing this, but we'll still need to show the expression.
• Config will need to be in the expression, not on the element probably, as the config will need to be accessible elsewhere for migration, etc?
• Input should still be updated as it changes through interaction (recentering a map, etc)

Resulting expression will be something like embeddable input={{key: value, key2: value2}}

[elasticmachine]

Pinging @elastic/kibana-presentation (Team:Presentation)