implement opa_eval to evaluate a query and store the result in a file

[ffortier]

Unlike opa_eval_binary that can only output to stdout, this variant is designed to output to a file that can be used in the build by other targets.

opa_eval(
    name = "check_bob_json",
    out = "bob.json",
    input = '{"name":"bob"}',
    query = "data.main.allow",
    deps = [":simple"],
)