Add optional ZONE parameter to ts-format

format-time-string accepts a third ZONE argument, which I think is necessary for formatting to a specific timezone

for example, I think the following is not possible currently with ts-format: `(format-time-string "%FT%TZ" (ts-unix (ts-now)) "UTC")`

adding a third optional ZONE parameter to ts-format as in format-time-string would fix this.