[xtask] Convert ci_download bash scripts to Rust (#5481)

Implements `cargo xtask download`, which provides options to replace the
existing scripts:

- ci_download_clickhouse
- ci_download_cockroachdb
- ci_download_console
- ci_download_dendrite_openapi
- ci_download_dendrite_stub
- ci_download_maghemite_mgd
- ci_download_maghemite_openapi
- ci_download_softnpu_machinery
- ci_download_thundermuffin
- ci_download_transceiver_control

This PR additionally introduces the `cargo xtask download all` option,
which attempts to download all artifacts concurrently.

Somewhat related to #3939

dev-tools/xtask/Cargo.toml

@@ -13,8 +13,20 @@ camino.workspace = true
 cargo_toml = "0.20"
 cargo_metadata.workspace = true
 clap.workspace = true
+flate2.workspace = true
+futures.workspace = true
 macaddr.workspace = true
+md5 = "0.7.0"
+reqwest = { workspace = true, features = [ "default-tls" ] }
 serde.workspace = true
+sha2.workspace = true
+slog.workspace = true
+slog-async.workspace = true
+slog-bunyan.workspace = true
+slog-term.workspace = true
+strum.workspace = true
+tar.workspace = true
+tokio = { workspace = true, features = ["full"] }
 toml.workspace = true
 fs-err.workspace = true
 swrite.workspace = true