separator functionality in API

[boromil]

when selecting multiple columns, SlashDB 0.9.23 has an ability to replace the default "," separator value using an additional query parameter - this should be reflected in our Angular SDK api
https://beta.slashdb.com/db/Chinook/Invoice/InvoiceId/1,2,3
can become
https://beta.slashdb.com/db/Chinook/Invoice/InvoiceId/1$2$3?separator=$