Core: revamp feed(???, Int) to produce arrays [BREAKING]

[slandelle]

Since early days of Gatling, we've supported a way to fetch multiple records at once.
However, the behavior is very awkward: the key are translated and suffixed with the 1 based index.

Instead, now that Gatling EL supports 0-basec index access, let's generate arrays instead, eg:

feed(Array(Map("foo" -> "bar"), Map("foo" -> "baz")), 2)

will inject

"foo" -> Array[Any]("bar", "baz")

which can be accessed with Gatling EL, eg #{foo(0).bar}