Add pre/post middleware support to command handlers

[wesen]

Adds support for pre and post middleware execution in command handlers:

• Add preMiddlewares and postMiddlewares fields to GenericCommandHandler
• Add WithPreMiddlewares() and WithPostMiddlewares() options
• Execute middleware chain in order: pre -> parameter filter -> post
• Fix middleware execution order in parameter filter
• Add extensive documentation for middleware patterns and usage
• Add parameter filter options for whitelist/blacklist configuration
• Fix template lookup initialization
• Add error handling for handler initialization

Breaking changes:

• NewCommandHandler() now returns error
• NewGenericCommandHandler() now returns error
• Parameter filter middleware execution order changed