Use a structured magnet URL type

This will be required to parse them and submit them to bitmagnet.

Cargo.lock

@@ -1154,6 +1154,16 @@ version = "0.4.27"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "13dc2df351e3202783a1fe0d44375f7295ffb4049267b0f3018346dc122a1d94"
 
+[[package]]
+name = "magnet-url"
+version = "2.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ac3b4c4004e88aca00cc0c60782e5642c8fc628deca19e530ce58aa76e737d74"
+dependencies = [
+ "lazy_static",
+ "regex",
+]
+
 [[package]]
 name = "maybe-async"
 version = "0.2.10"
@@ -1538,6 +1548,7 @@ dependencies = [
  "figment",
  "figment_file_provider_adapter",
  "log",
+ "magnet-url",
  "multimap",
  "pretty_env_logger",
  "regex",