ftl: contains

ftl::contains(container, value) returns true if the container contains
the value. It's implemented in terms of std::find(), so has the same
runtime complexity.

It is otherwise a simplified version of the C++23 std::ranges::contains,
which has some additional options, and can be deprecated once C++23 is
the minimal supported version.

Test: atest ftl_test
Bug: 185536303
Change-Id: I98aefe7cf6645ac3a20fddfe0657fa6822d669de