Restrict target_keys object to contain only a single key value pair.

[annakrystalli]

This PR resolves #117 by restricting target_keys objects to contain only a single key value pair.

Successful validation is demonstrated in hubAdmin PR hubverse-org/hubAdmin#90 and more specifically in these tests using this and this test file.

Question

Should we also remove target_variable and target_outcome from the schema? These were originally added as standard task ids (and are documented as such in hubDocs) for the following reason:

target_variable/target_outcome: task IDs making up unique identifiers of a two-part target. These tasks can be used in hubs that want to divide the definition of a target across two variables. Both task IDs will be specified as target keys in the target_metadata array.

The answer also therefore has implications on hubverse-org/hubDocs#226