generator: add optional type generator for user-defined MessagePack types

[bigbes]

Add infrastructure to generate optional wrapper types for custom types that
implement MessagePack extension methods (MarshalMsgpack/UnmarshalMsgpack).

Changes:

• New CLI tool (cmd/gentypes) to generate code for specific types.
• Support for MessagePack extension format with configurable extension codes.
• Compatibility with existing option package patterns.
• Updated linter exclusions for generator code paths.
• Added import allow lists for new dependencies.

Example usage:
go run cmd/gentypes -ext-code 1 -package ./path/to/package TypeName

Closes #TNTP-3734.

[coveralls]

coverage: 91.616%. remained the same
when pulling 22ee789 on bigbes/tntp-3734-optional-types-generator
into 7a7e26c on master.

[oleg-jukovec]

The commit message ideally should be short (<52 chars), so you need to update it:

generator: add optional type generator for user-defined MessagePack types

->

generator: add generator for user-defined types

[oleg-jukovec]

Thank you for the patch.

To be honest, it is unclear how to use this now and why it is needed.

Please add a description in the README.md for this generator and how to use it at this PR.

[oleg-jukovec]

Please, test a public API only.

Suggested change

	package extractor
	package extractor_test

[bigbes]

fixed

[Copilot]

Pull Request Overview

This PR adds infrastructure for generating optional wrapper types for custom types that implement MessagePack extension methods. It introduces a new CLI tool to generate code for specific types with configurable MessagePack extension codes and maintains compatibility with existing option package patterns.

• Adds cmd/gentypes CLI tool for generating optional types with MessagePack extension support
• Implements code generation templates for both optional types and their corresponding tests
• Creates test infrastructure including the FullMsgpackExtType example with complete MessagePack serialization

Reviewed Changes

Copilot reviewed 20 out of 21 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
go.mod	Updates dependencies to support the new code generation tool
cmd/gentypes/generator.go	Main CLI application for generating optional types
cmd/gentypes/generator/extension.go	Core code generation logic and template execution
cmd/gentypes/generator/type_gen.go.tpl	Template for generating optional type implementations
cmd/gentypes/generator/type_gen_test.go.tpl	Template for generating comprehensive test suites
cmd/gentypes/extractor/*.go	Package analysis and AST extraction utilities
cmd/gentypes/test/*.go	Example implementation and generated test files
README.md	Documentation for the new gentype utility
.golangci.yml	Updated linter configuration for generator code paths

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[patapenka-alexey]

Looks like all dots in the right places.