Fabric settings `max_*_routes` - need a "don't care" option

[chrismarget-j]

Each of the max_*_routes elements in the FabricSettings{} struct has 2 possible states:

• nil pointer, in which case we send null to to clear the current value (the omitempty struct tag is not configured)
• non-nil pointer, in which case we send a numeric value

There's no way for a caller to indicate whether they intend to clear the current value.

One possible option:

• Change the type in the public and private structs from *uint32 to *int64
• Give special meaning to -1 (affirm clearing the value)
• Write a MarshalJson method which determines whether the values will be sent based on the -1

This approach is complicated by the need to support Apstra 4.2.1, 4.2.0 and 4.1.x, which each handle this data a bit differently.