As a user, I want to store other external Study ID numbers in the CTML

[jagnathan]

As a user, I want to store other Study ID numbers in the CTML.
In clinicaltrials.gov website, it states the following:
Identifiers or ID numbers other than the NCT number that are assigned to a clinical study by the study's sponsor, funders, or others. These numbers may include unique identifiers from other trial registries and National Institutes of Health grant numbers.
Example:
CAPTUR trial has a unique NCT Trial ID.
It also has the following Study IDs.
PM1
ESR-17-12831 ( Other Identifier ) (OTHER: AstraZeneca)
CA209-9DL ( Other Identifier ) (OTHER: Bristol-Myers Squibb)
ML39800 ( Other Identifier ) (OTHER: Hoffmann-La Roche)
WI233446 ( Other Identifier ) (OTHER: Pfizer)
Similarly, UHN will have CAPCR number, REB number, etc. corresponding to other systems of UHN.
We can have a list of Identifiers which are stored as a key-value pair. There wont be any constraint on this as both the key and value will be stored as a set of key-value pairs in the CTML.
This is metadata information and will not be used in the matching criteria.
ExternalIDs:
    - ID:
        key:AstraZeneca
        Value:ESR-17-12831
    - ID:
        key:Bristol-Myers Squibb
        Value:CA209-9DL