Refactor CommandsExecutor - is no longer static singleton

Preparing for change in Command#handle,
which is going to take a CommandContext as an additional argument.
This way we can pass in command dependencies in a safe way
instead of requiring them to access static methods.

src/main/java/client/command/CommandContext.java

@@ -0,0 +1,6 @@
+package client.command;
+
+import database.drop.DropProvider;
+
+public record CommandContext(DropProvider dropProvider) {
+}