New get_metadata() plugin hook for dynamic metadata

The following hook is added:

    get_metadata(
      datasette=self, key=key, database=database, table=table,
      fallback=fallback
    )

This gets called when we're building our metdata for the rest
of the system to use. We merge whatever the plugins return
with any local metadata (from metadata.yml/yaml/json) allowing
for a live-editable dynamic Datasette.

As a security precation, local meta is *not* overwritable by
plugin hooks. The workflow for transitioning to live-meta would
be to load the plugin with the full metadata.yaml and save.
Then remove the parts of the metadata that you want to be able
to change from the file.

* Avoid race condition: don't mutate databases list

This avoids the nasty "RuntimeError: OrderedDict mutated during
iteration" error that randomly happens when a plugin adds a
new database to Datasette, using `add_database`. This change
makes the add and remove database functions more expensive, but
it prevents the random explosion race conditions that make for
confusing user experience when importing live databases.

Thanks, @brandonrobertz

docs/plugin_hooks.rst

@@ -1129,3 +1129,38 @@ This example will disable CSRF protection for that specific URL path:
         return scope["path"] == "/submit-comment"
 
 If any of the currently active ``skip_csrf()`` plugin hooks return ``True``, CSRF protection will be skipped for the request.
+
+get_metadata(datasette, key, database, table, fallback)
+-------------------------------------------------------
+
+``datasette`` - :ref:`internals_datasette`
+    You can use this to access plugin configuration options via ``datasette.plugin_config(your_plugin_name)``, or to execute SQL queries.
+
+``actor`` - dictionary or None
+    The currently authenticated :ref:`actor <authentication_actor>`.
+
+``database`` - string or None
+    The name of the database metadata is being asked for.
+
+``table`` - string or None
+    The name of the table.
+
+``key`` - string or None
+    The name of the key for which data is being asked for.
+
+This hook is responsible for returning a dictionary corresponding to Datasette :ref:`metadata`. This function is passed the `database`, `table` and `key` which were passed to the upstream internal request for metadata. Regardless, it is important to return a global metadata object, where `"databases": []` would be a top-level key. The dictionary returned here, will be merged with, and overwritten by, the contents of the physical `metadata.yaml` if one is present.
+
+.. code-block:: python
+
+    @hookimpl
+    def get_metadata(datasette, key, database, table, fallback):
+        metadata = {
+            "title": "This will be the Datasette landing page title!",
+            "description": get_instance_description(datasette),
+            "databases": [],
+        }
+        for db_name, db_data_dict in get_my_database_meta(datasette, database, table, key):
+            metadata["databases"][db_name] = db_data_dict
+        # whatever we return here will be merged with any other plugins using this hook and
+        # will be overwritten by a local metadata.yaml if one exists!
+        return metadata