Expose integrate_1d variadic signatures

[andrjohns]

Description

Discussed over in the stanc3 repo, integrate_1d_impl supports variadic arguments, but this is not exposed by the user-facing integrate_1d function.

Current Version:

v4.6.2

[wds15]

Bummer that is not yet in! I just ran into the need for this just now... would be awesome if this moves forward.