Support pulling/pushing VMs to OCI-compatible registries (#32)

* Support pulling/pushing VMs to OCI-compatible registries

* Registry: rename request() to endpointRequest() for clarity

* Registry: include JSON details on HTTP status code mismatch errors

* .cirrus.yml: run tests

* Fix testDigest

* Registry: set Content-{Length,Type} headers when pushing blob

* Refactor Registry.auth() and enrich RegistryError.AuthFailed

* Remove useless comment

* Fix WWWAuthenticate a bit and add tests

* WWWAuthenticate: expect a Bearer scheme

* Registry.auth(): document the passing of ["scope", "service"] parameters

* Clarify unexpected HTTP code error when retrieving auth token

* Make RemoteName parser more relaxed for now

* tart clone: pull the VM if it's OCI-based first

* VMStorageOCI: ensure the old symbolic link is overwritten

* tart push: support multiple remote VM names

* Logging: push/pull progress

* Use 500 MB chunks (instead of 500 MiB) to evenly cut disk

...which also uses powers of 10.

* Credentials: only read credentials labeled "Tart Credentials"

Package.swift

@@ -12,11 +12,13 @@ let package = Package(
   ],
   dependencies: [
     .package(url: "https://github.com/apple/swift-argument-parser", from: "1.1.2"),
+    .package(url: "https://github.com/pointfreeco/swift-parsing", from: "0.9.2"),
   ],
   targets: [
-    .executableTarget(name: "tart",
-      dependencies: [
-        .product(name: "ArgumentParser", package: "swift-argument-parser"),
-      ]),
+    .executableTarget(name: "tart", dependencies: [
+      .product(name: "ArgumentParser", package: "swift-argument-parser"),
+      .product(name: "Parsing", package: "swift-parsing"),
+    ]),
+    .testTarget(name: "TartTests", dependencies: ["tart"])
   ]
 )