Add plugin update checking and a plugins command (#898)

* Add plugin update checking

Report available updates to the installed MVT plugin packages in the
startup banner, for plugins installed from a package index and for
plugins installed directly from a repository. Repository installs pinned
to a commit or a tag are never reported as outdated.

MVT only prints the command which upgrades a plugin. Installing the
update stays a deliberate choice of the analyst. The check runs at most
once every twelve hours, and in between prints the findings of the
latest check which still apply to what is installed.

Nothing about the check can interrupt a running command: the parts of
the suggested command come from package metadata and are quoted for the
shell, the repository query refuses to prompt for credentials and never
passes metadata as a git option, and a corrupt or stale cache is
discarded rather than trusted.

* Add a plugins command to list installed plugins and check updates

Add a "plugins" command to the platform-neutral mvt command. "plugins
list" shows every installed plugin package with its version, where it was
installed from, how many forensic modules it contributes and which
commands it adds. "plugins check-updates" checks for updates immediately,
without waiting for the automatic check, and prints the command which
upgrades a plugin instead of installing anything.

It lives on mvt only. The packages it lists extend mvt-ios and mvt-android
too, but auditing them is not the job of a command which analyses one
platform, and the two platform CLIs should not carry commands which are
not about an acquisition.

The command is registered as a built-in, before any external command, so
that an installed package cannot replace this audit surface.

docs/development/custom_commands.md

@@ -60,7 +60,8 @@ pipx inject mvt my-mvt-plugin
 ```
 
 When MVT is installed in an active virtual environment, install the plugin with
-`pip` in that environment.
+`pip` in that environment. `mvt plugins list` shows the installed packages and
+the commands they add, see [Managing Plugins](plugins.md).
 
 Command packages that need their own settings, such as an API key, should store
 them in a namespaced [plugin configuration file](plugin_configuration.md)
@@ -69,9 +70,9 @@ rather than in MVT's own `config.yaml`.
 ### Commands on `mvt`
 
 The `mvt` command hosts what belongs to neither platform: `version`,
-`completion` and `download-iocs`. A plugin command which is not about the
-acquisition of one platform, such as one which configures the plugin or
-synchronizes the indicators it uses, belongs there too, in the
+`completion`, `plugins` and `download-iocs`. A plugin command which is not
+about the acquisition of one platform, such as one which configures the plugin
+or synchronizes the indicators it uses, belongs there too, in the
 `mvt.cli_plugins` group:
 
 ```toml