lib-http: rename http_header_field.key to name

lib-http: Removed "pure" attribute from http_header_field_find_unique(). It's a bit questionable if returning a pointer via parameter is allowed for a pure function. Also having this attribute seems to confuse clang static analyzer, because it gives false positives for code using http_header_field_find_unique(). It's not important for performance anyway, so better to avoid using it.

lib-http: Implemented limits on overall HTTP header size, size of individual header fields and the number of fields in the header.

lib-http: Unified http-request.h and http-response.h headers. Renamed struct http_response_header to struct http_header_field, encapsulated the array in struct http_header and put it all in http-header.h/c Added inline utility functions for header querying and getting response/request payload size.