bindings: rust: make request_config optional in Chip.request_lines()

bindings: rust: make request_config optional in Chip.request_lines()

Request config is not necessary to request lines. In C API we accept
a NULL pointer, in C++ it's not necessary to assign a request_config
to the request builder, in Python the consumer and event buffer size
arguments are optional. Let's make rust bindings consistent and not
require the request config to be always present. Convert the argument
in request_lines to Option and update the rest of the code.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>