GDExtension interface: Add `is_abstract` to `GDExtensionClassMethodInfo`

[charjr]

Tested versions

4.5.1

Issue description

When using gdextension_interface.h an abstract class can be created via

typedef struct {
        ...
	GDExtensionBool is_abstract;
	...
} GDExtensionClassCreationInfo4;

An abstract method cannot be created.

typedef enum {
	GDEXTENSION_METHOD_FLAG_NORMAL = 1,
	GDEXTENSION_METHOD_FLAG_EDITOR = 2,
	GDEXTENSION_METHOD_FLAG_CONST = 4,
	GDEXTENSION_METHOD_FLAG_VIRTUAL = 8,
	GDEXTENSION_METHOD_FLAG_VARARG = 16,
	GDEXTENSION_METHOD_FLAG_STATIC = 32,
	GDEXTENSION_METHOD_FLAGS_DEFAULT = GDEXTENSION_METHOD_FLAG_NORMAL,
} GDExtensionClassMethodFlags;

...

typedef struct {
	GDExtensionStringNamePtr name;
	void *method_userdata;
	GDExtensionClassMethodCall call_func;
	GDExtensionClassMethodPtrCall ptrcall_func;
	uint32_t method_flags;
	GDExtensionBool has_return_value;
	GDExtensionPropertyInfo *return_value_info;
	GDExtensionClassMethodArgumentMetadata return_value_metadata;
	uint32_t argument_count;
	GDExtensionPropertyInfo *arguments_info;
	GDExtensionClassMethodArgumentMetadata *arguments_metadata;
	uint32_t default_argument_count;
	GDExtensionVariantPtr *default_arguments;
} GDExtensionClassMethodInfo;

Steps to reproduce

godot --dump-extension-interface

[AThousandShips]

The missing flag seems to be METHOD_FLAG_VIRTUAL_REQUIRED from the engine side