Change beaconFilteringTermsResults type to enum

[costero-e]

This PR is for the issue opened by Sergi (I can't tag Sergi) here #77 . Please @mrueda and @redmitry, can you review this PR? Thank you.

[redmitry]

when type is "ontology", "id" prefix should be used to find the ontology in resources.

[jrambla]

This PR is a fix, however it is breaking the specification and potentially some implementations.
DO we have a policy for such cases?

[mbaudis]

IMO good & doesn't break implementations since the values stay string etc. (but possibly breaks verifiers if not updated).
Small change suggested: Style-wise we always should do plural examples: [].

[jrambla]

@mbaudis, I believe we must consider a change as breaking if the current implementations are compliant with the current spec, BUT the change on the spec makes them not compliant anymore, Which is this case, e.g. teh Beacon Network will reject the filteringTerms endpoint response as invalid ;-)

[mbaudis]

@jrambla Sure, understood. But I guess the amount of aggregators, and esp. of the ones doing such checks, outside of Beacon core developer control are ... rather small. But anyway a change that can wait.