Add overload-max-heap field in contourconfig

[yangyy93]

Please describe the problem you have
[A clear, concise, description of the problem you are facing. What is the problem that feature X would solve for you?]
I want to use Overload Manager, but contourconfig doesn’t have this field. Is it possible to add a field overload-max-heap? If this field exists when configuring the envoy-initconfig container, add the startup args --overload-max-heap

[sunjayBhatia]

This seems reasonable to add, will add the help wanted label on this