It should be possible to add 'external IDs' (suggestions for better term are welcome!) to an existing anchor.

1. External ID is a key-value pair of 2 strings (probably)
2. An anchor can have many 'external ID's
   1. implement as Map<String, String> on Anchor Object
3. CPS does not enforce uniqueness of external ID values ie. several anchors in one dataspace can have the same external ID (its probably not to be expected but not the responsibility of CPS to check this)
4. TBD: Allow update of the same external key? See discussion on being idempotent here https://wiki.onap.org/pages/viewpage.action?pageId=92998259 #5
5. TBD: Delete of key not needed (if so separate user story)

Implementation (this user story) should include

1. REST endpoint
2. Java API 
3. DB SPI
4. DB Schema