Support for `external` predicates.

[jcreedcmu]

Add ability to run queries with predicates marked external, and ability to use prior query runs as input data for external predicates. This would allow very flexible custom notions of comparison and filtering of previous query results.

[aeisenberg]

Do you have any more information on how this would work?

[jcreedcmu]

This is related to how jump-to-definition already works, since https://github.com/github/codeql/blob/master/javascript/ql/src/localDefinitions.ql has an external predicate in it.

But the way the extension interacts with it is by already having baked in which external predicate it's going to supply data for --- I think if I remember right the idea of this issue was to let the user come across an arbitrary query with external predicates in it and provide some kind of interface for choosing data to put into them. https://help.semmle.com/QL/ql-handbook/annotations.html#external