key: move add, list, remove, passwd to sub-commands

docs: improve the sub-command docs

changelog: add the unreleased changelog for the key command updates

key: update integration tests

changelog/unreleased/issue-4676

@@ -0,0 +1,8 @@
+Enhancement: Move key add, list, remove and passwd as separate sub-commands
+
+Restic now provides usage documentation for the `key` command. Each sub-command;
+`add`, `list`, `remove` and `passwd` now have their own sub-command documentation
+which can be invoked using `restic key <add|list|remove|passwd> --help`.
+
+https://github.com/restic/restic/issues/4676
+https://github.com/restic/restic/pull/4685