Metadata API to indicate cluster member specificity

[edlerd]

Required information

• Distribution: snap
• Distribution version: 6.1

Issue description

The GET 1.0/metadata/configuration response should include an indicator if a field is global or cluster member specific, when LXD is clustered. This information should also be added to the documentation.

For instance, the key config.network-physical.network-conf.keys[18].parent has no notion of being specific per cluster member. When LXD is clustered, the key cannot be set without targetting a specific cluster member. See screenshot below for its current value.