LibAudio: Add a generic audio metadata container

This container has several design goals:
- Represent all common and relevant metadata fields of audio files in a
  unified way.
- Allow perfect recreation of any metadata format from the in-memory
  structure. This requires that we allow non-detected fields to reside
  in an "untyped" miscellaneous collection.

Like with pictures, plugins are free to store their metadata into the
m_metadata field whenever they read it. It is recommended that this
happens on loader creation; however failing to read metadata should not
cause an error in the plugin.

Userland/Libraries/LibAudio/Metadata.cpp

@@ -0,0 +1,94 @@
+/*
+ * Copyright (c) 2023, kleines Filmröllchen <filmroellchen@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "Metadata.h"
+#include <AK/Assertions.h>
+#include <LibCore/Version.h>
+
+namespace Audio {
+
+bool Person::is_artist() const
+{
+    return role == Person::Role::Artist
+        || role == Person::Role::Composer
+        || role == Person::Role::Conductor
+        || role == Person::Role::Lyricist
+        || role == Person::Role::Performer;
+}
+
+Optional<StringView> Person::name_for_role() const
+{
+    switch (role) {
+    case Role::Artist:
+    case Role::Performer:
+        return {};
+    case Role::Lyricist:
+        return "Lyricist"sv;
+    case Role::Conductor:
+        return "Conductor"sv;
+    case Role::Publisher:
+        return "Publisher"sv;
+    case Role::Engineer:
+        return "Engineer"sv;
+    case Role::Composer:
+        return "Composer"sv;
+    }
+    VERIFY_NOT_REACHED();
+}
+
+void Metadata::replace_encoder_with_serenity()
+{
+    auto version_or_error = Core::Version::read_long_version_string();
+    // Unset the encoder field in this case; we definitely want to replace the existing encoder field.
+    if (version_or_error.is_error())
+        encoder = {};
+    auto encoder_string = String::formatted("SerenityOS LibAudio {}", version_or_error.release_value());
+    if (encoder_string.is_error())
+        encoder = {};
+    encoder = encoder_string.release_value();
+}
+
+Optional<String> Metadata::first_artist() const
+{
+    auto artist = people.find_if([](auto const& person) { return person.is_artist(); });
+    if (artist.is_end())
+        return {};
+    return artist->name;
+}
+
+ErrorOr<Optional<String>> Metadata::all_artists(StringView concatenate_with) const
+{
+    // FIXME: This entire function could be similar to TRY(TRY(people.filter(...).try_map(...)).join(concatenate_with)) if these functional iterator transformers existed :^)
+    Vector<String> artist_texts;
+    TRY(artist_texts.try_ensure_capacity(people.size()));
+    for (auto const& person : people) {
+        if (!person.is_artist())
+            continue;
+        if (auto role_name = person.name_for_role(); role_name.has_value())
+            artist_texts.unchecked_append(TRY(String::formatted("{} ({})", person.name, role_name.release_value())));
+        else
+            artist_texts.unchecked_append(person.name);
+    }
+    if (artist_texts.is_empty())
+        return Optional<String> {};
+    return String::join(concatenate_with, artist_texts);
+}
+
+ErrorOr<void> Metadata::add_miscellaneous(String const& field, String value)
+{
+    // FIXME: Since try_ensure does not return a reference to the contained value, we have to retrieve it separately.
+    //        This is a try_ensure bug that should be fixed.
+    (void)TRY(miscellaneous.try_ensure(field, []() { return Vector<String> {}; }));
+    auto& values_for_field = miscellaneous.get(field).release_value();
+    return values_for_field.try_append(move(value));
+}
+
+ErrorOr<void> Metadata::add_person(Person::Role role, String name)
+{
+    return people.try_append(Person { role, move(name) });
+}
+
+}