Add language-specific code generation options to the generated OpenAPI .proto about gnostic

[timburks]

Follow up question:

Should the Java package name be com.openapi.v2, org.openapi.v2, or something else?

[timburks]

I propose using the following values:

// This option lets the proto compiler generate Java code inside the package
// name (see below) instead of inside an outer class. It creates a simpler
// developer experience by reducing one-level of name nesting and be
// consistent with most programming languages that don't support outer classes.
option java_multiple_files = true;

// The Java outer classname should be the filename in UpperCamelCase. This
// class is only used to hold proto descriptor, so developers don't need to
// work with it directly.
option java_outer_classname = "OpenAPIProto";

// The Java package name must be proto package name with proper prefix.
option java_package = "org.openapi.v2";

// A reasonable prefix for the Objective-C symbols generated from the package.
// It should at a minimum be 3 characters long, all uppercase, and convention
// is to use an abbreviation of the package name. Something short, but
// hopefully unique enough to not conflict with things that may come along in
// the future. 'GPB' is reserved for the protocol buffer implementation itself.
//
option objc_class_prefix = "OAS"; // "OpenAPI Spec"

[timburks]

Done in this commit.