JS API must support SKIP as an aggregation type

[niloc132]

Fixes #4182

[mofojed]

Looks good, tested with the steps in the ticket #4182 but with this snippet specifying the defaultOperation:

// `temp1` should be the name of your global variable the table is assigned to
var totalsTable = await temp1.getTotalsTable({ defaultOperation: 'Skip', operationMap: { I: ['Sum', 'Count'] } })
console.log('totals columns: ', totalsTable.columns.map(c => c.name))

I do find it slightly odd that our default operation is Sum instead of Skip, but that seems to be consistent with Enterprise.