Various improvements to plugin metadata

- Add plugin (display) name and plugin URL
 - Make everything except plugin ID optional (instead of empty string)
 - Exclude empty properties from generated velocity-plugin.json
 - Make plugin author list immutable
 - Other (minor) cleanup and refactoring

api/src/main/java/com/velocitypowered/api/plugin/Plugin.java

@@ -21,6 +21,14 @@ public @interface Plugin {
      */
     String id();
 
+    /**
+     * The human readable name of the plugin as to be used in descriptions and
+     * similar things.
+     *
+     * @return The plugin name, or an empty string if unknown
+     */
+    String name() default "";
+
     /**
      * The version of the plugin.
      *
@@ -28,6 +36,13 @@ public @interface Plugin {
      */
     String version() default "";
 
+    /**
+     * The URL or website of the plugin.
+     *
+     * @return The plugin url, or an empty string if unknown
+     */
+    String url() default "";
+
     /**
      * The author of the plugin.
      *

api/src/main/java/com/velocitypowered/api/plugin/PluginDescription.java

@@ -1,5 +1,6 @@
 package com.velocitypowered.api.plugin;
 
+import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableSet;
 import com.velocitypowered.api.plugin.meta.PluginDependency;
 
@@ -7,7 +8,6 @@ import java.nio.file.Path;
 import java.util.Collection;
 import java.util.List;
 import java.util.Optional;
-import java.util.Set;
 import java.util.regex.Pattern;
 
 /**
@@ -29,21 +29,45 @@ public interface PluginDescription {
      */
     String getId();
 
+    /**
+     * Gets the name of the {@link Plugin} within this container.
+     *
+     * @return an {@link Optional} with the plugin name, may be empty
+     * @see Plugin#name()
+     */
+    default Optional<String> getName() {
+        return Optional.empty();
+    }
+
     /**
      * Gets the version of the {@link Plugin} within this container.
      *
-     * @return the plugin version
+     * @return an {@link Optional} with the plugin version, may be empty
      * @see Plugin#version()
      */
-    String getVersion();
+    default Optional<String> getVersion() {
+        return Optional.empty();
+    }
+
+    /**
+     * Gets the url or website of the {@link Plugin} within this container.
+     *
+     * @return an {@link Optional} with the plugin url, may be empty
+     * @see Plugin#url()
+     */
+    default Optional<String> getUrl() {
+        return Optional.empty();
+    }
 
     /**
      * Gets the authors of the {@link Plugin} within this container.
      *
-     * @return the plugin authors
+     * @return the plugin authors, may be empty
      * @see Plugin#authors()
      */
-    List<String> getAuthors();
+    default List<String> getAuthors() {
+        return ImmutableList.of();
+    }
 
     /**
      * Gets a {@link Collection} of all dependencies of the {@link Plugin} within

api/src/main/java/com/velocitypowered/api/plugin/meta/PluginDependency.java

@@ -3,6 +3,7 @@ package com.velocitypowered.api.plugin.meta;
 import javax.annotation.Nullable;
 
 import java.util.Objects;
+import java.util.Optional;
 
 import static com.google.common.base.Preconditions.checkArgument;
 import static com.google.common.base.Preconditions.checkNotNull;
@@ -36,11 +37,10 @@ public final class PluginDependency {
     /**
      * Returns the version this {@link PluginDependency} should match.
      *
-     * @return the plugin version, or {@code null} if unspecified
+     * @return an {@link Optional} with the plugin version, may be empty
      */
-    @Nullable
-    public String getVersion() {
-        return version;
+    public Optional<String> getVersion() {
+        return Optional.ofNullable(version);
     }
 
     /**