gentypes: add support for third-party types

[bigbes]

The gentypes tool was previously limited to generating optional types for local
types that implemented the MarshalMsgpack and UnmarshalMsgpack methods. Thi
made it impossible to generate optional types for types from external modules.

This commit enhances the generator to support third-party types by introducing
the following new flags:

• -force: To bypass the check for MarshalMsgpack and UnmarshalMsgpack methods.
• -imports: To add necessary imports for the third-party type and custom functions.
• -marshal-func: To specify a custom marshal function.
• -unmarshal-func: To specify a custom unmarshal function.

The generator code has been refactored to use a GenerateOptions struct for
better organization.

Additionally, this commit:

• Adds a new test case for generating an optional type for uuid.UUID.
• Updates the README.md with documentation for the new flags and an example.
• Moves test files to a more appropriate internal/test directory.

Closes #TNTP-3734.

[coveralls]

coverage: 91.616%. remained the same
when pulling ecdf7af on bigbes/tntp-3734-gentypes-force
into 54ff5d9 on master.

[oleg-jukovec]

We can actually add these methods if there is a reason for it. We have already been asked for these features:

tarantool/go-tarantool#421

[Copilot]

Pull Request Overview

This PR extends the gentypes tool to support generating optional types for third-party types. Previously, the tool was limited to generating optional types for local types that implemented MarshalMsgpack and UnmarshalMsgpack methods.

• Adds support for third-party types with new command-line flags (-force, -imports, -marshal-func, -unmarshal-func)
• Refactors the generator to use a GenerateOptions struct for better organization
• Includes comprehensive test coverage for uuid.UUID as an example third-party type

Reviewed Changes

Copilot reviewed 14 out of 17 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
cmd/gentypes/main.go	Adds new CLI flags and logic for handling third-party types
cmd/gentypes/generator/extension.go	Refactors GenerateByType to use GenerateOptions struct
cmd/gentypes/generator/type_gen.go.tpl	Updates template to support custom marshal/unmarshal functions
cmd/gentypes/flag.go	Implements stringListFlag type for handling multiple imports
cmd/gentypes/internal/test/uuid_gen.go	Generated optional type for uuid.UUID as test case
README.md	Documents new flags and provides usage examples
go.mod	Adds github.com/google/uuid dependency

---

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

[bigbes]

@oleg-jukovec I've redone this logic once again :( now it has more examples and has even better support for third-party libraries.