Add overloads to Flow.sample to allow sampling multiple flows.

We have an increasing number of flows that sample a combination of
several other flows, which results in extra tuple transform boilerplate:

a.sample(combine(b, c, d, ::Triple), ::toQuad).collect {
    (a, b, c, d) -> ...

Adding another flow also requires updating both transforms. This
cleans that up:

a.sample(b, c, d).collect {
    (a, b, c, d) -> ...

Bug: 278086361
Test: N/A
Flag: N/A
Change-Id: Ic71ea6a23a4138fb8989e9564b0f93d7ae916c74