migrate mg-admin and ddm-admin APIs to Dropshot API manager (#557)

Part of oxidecomputer/omicron#8922.

Also added an xtask so users can run `cargo xtask openapi generate` and so on.

Author: sunshowers

.cargo/config.toml

@@ -1,6 +1,9 @@
 [net]
 git-fetch-with-cli = true
 
+[alias]
+xtask = "run --package xtask --"
+
 [build]
 rustflags = ["--cfg", "tokio_unstable"]