incusd: Support incus as a protocol #24

Job	Run time
Code tests	9m 34s
Client tests (1.18.x, ubuntu-latest)	29s
Client tests (1.18.x, macos-latest)	45s
Client tests (1.18.x, windows-latest)	2m 22s
System tests (1.18.x, cluster, dir)	10m 17s
System tests (1.18.x, cluster, btrfs)	10m 38s
System tests (1.18.x, cluster, lvm)	11m 15s
System tests (1.18.x, cluster, zfs)	11m 23s
System tests (1.18.x, cluster, ceph)	14m 59s
System tests (1.18.x, cluster, random)	11m 1s
System tests (1.18.x, standalone, dir)	4m 28s
System tests (1.18.x, standalone, btrfs)	5m 4s
System tests (1.18.x, standalone, lvm)	6m 46s
System tests (1.18.x, standalone, zfs)	5m 57s
System tests (1.18.x, standalone, ceph)	11m 54s
System tests (1.18.x, standalone, random)	6m 50s
System tests (stable, cluster, dir)	10m 52s
System tests (stable, standalone, dir)	6m 14s
System tests (tip, cluster, dir)	11m 22s
System tests (tip, standalone, dir)	5m 21s
Documentation tests	8m 23s
	2h 45m 54s

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