Configure large_client_header_buffers nginx directive via APICast CRD and APIManager CRD

We want to be able to increase the limit of http://nginx.org/en/docs/http/ngx_http_core_module.html#large_client_header_buffers in http1.1. The default is 4 x 8kB at the moment.

We have large request URIs with GET parameters and need to be able to handle them in APICast.

The request is to make this configurable globally for self-managed APICast via the operator.