Will return a string representation of only the possible values that match the value prefix for this clause.
This should not return more results than specified in maxNumResults. If it is possible this should use the
maxNumResults to efficiently generate the results.
The contract of this method is that if the valuePrefix exactly (minus case) matches the suggestion then we suggest it.
This will allow users to verify in their own minds that even though they have typed
the full value, it is still valid.
jqlClauseName - the jql clause name that was entered by the user, represents the identifier that was used
to find this values generator. Note: for custom fields this can be used to identify the custom field we are
valuePrefix - the portion of the value that has already been provided by the user.
maxResults - the maximun number of results to return.
a string value of the clause values that match the provided value prefix, empty list if none