Expose key flags on `DataCenterNode`

[alexrp]

novadrop/src/formats/Data/Serialization/Readers/DataCenterReader.cs

Lines 175 to 177 in 09d3a61

	// TODO: Should we allow setting 0b0001 in the API?
	if (keyFlags is not 0b0000 or 0b0001)
	throw new InvalidDataException($"Node has invalid key flags 0x{keyFlags:x1}.");

novadrop/doc/game/data-center-format.md

Lines 316 to 328 in 09d3a61

	`key_flags` is `0` in official data center files. It may have a combination of
	the following values:
	```cpp
	enum DataCenterKeyFlags : uint8_t
	{
	DATA_CENTER_KEY_FLAGS_NONE = 0b0000,
	DATA_CENTER_KEY_FLAGS_UNCACHED = 0b0001,
	};
	```
	If `DATA_CENTER_KEY_FLAGS_UNCACHED` is set, the results of a query against this
	node will not be cached by the client.

[alexrp]

As of #21, we can both consume and produce deduplicated data center files, so general memory usage is down quite a bit. That fact makes this issue more palatable since adding an extra field to DataCenterNode is no longer so bad.

That said, need to think about how this will be represented in XML form.

[alexrp]

In XML form, we will likely just have a dc:flags attribute in the XSD, like we have dc:keys.