common/schema: turns into a component

This is a first step to make it accept configuration. Most of the
changes are quite trivial, but I also ran into some difficulties with
query columns and filters. They need the schema for parsing, but parsing
happens before dependencies are instantiated (and even if it was not the
case, parsing is stateless). Therefore, I have added a `Validate()`
method that must be called after instantiation. Various bits `panic()`
if not validated to ensure we catch all cases.

The alternative to make the component manages a global state would have
been simpler but it would break once we add the ability to add or
disable columns.

common/schema/root.go

@@ -0,0 +1,19 @@
+// SPDX-FileCopyrightText: 2023 Free Mobile
+// SPDX-License-Identifier: AGPL-3.0-only
+
+// Package schema is an abstraction of the data schema for flows used by
+// Akvorado. It is a leaky abstraction as there are multiple parts dependant of
+// the subsystem that will use it.
+package schema
+
+// Component represents the schema compomenent.
+type Component struct {
+	Schema
+}
+
+// New creates a new schema component.
+func New() (*Component, error) {
+	return &Component{
+		Schema: flows(),
+	}, nil
+}