select Verification Method
Select verification method takes a selector that can be used to select a specific verification method from the DID Document. If a selector is not provided, the first verification method is returned
The selector can either be an ID, Purpose, or null. If a Purpose is provided, the first verification method in the DID Document that has the provided purpose will be returned.
Return
VerificationMethod matching the selector criteria
Parameters
selector
can either be an ID, Purpose, or null