Refactor: transparent and extensible cuegen decl

[iyear]

Description of your changes

🤖 Generated by Copilot at b3c4005

Summary

🧪🏗️♻️

This pull request refactors the CUE generator code to use a new Decl interface for different kinds of CUE declarations, such as structs, and adds a Struct type that implements the interface. This simplifies the code structure, makes it more extensible, and enables common fields and methods for CUE declarations. The pull request also updates the test cases and modifies the provider.go file to use the new interface.

We're the CUE generator crew, we make the code for you
We use the Decl interface to handle any case
We build the structs and fields with common methods and skills
We heave away and haul away and format them with grace

Walkthrough

• Introduce a new interface Decl and a new type Struct that implement it in decl.go (link, link)
• Change the return type of Generate and convertDecls methods from []cueast.Decl to []Decl in convert.go and generator.go (link, link)
• Change the parameter type of Format and modifyDecls functions from []cueast.Decl to []Decl in generator.go and provider.go (link, link)
• Refactor the logic of convertDecls to use a switch statement instead of an if statement for handling different kinds of underlying types in convert.go (link)
• Modify the logic of Format to call the Build method of each Decl and append the result to the f.Decls slice in generator.go (link)
• Modify the logic of modifyDecls to use a type assertion to check if each Decl is a Struct and use its fields to populate the mapping map in provider.go (link)
• Modify the logic of modifyDecls to create and append a Struct instance instead of a cueast.Field instance to the decls slice in provider.go (link)
• Remove the unused import of cueast in generator_test.go (link)
• Change the parameter type and the argument type of the Format method calls in TestGeneratorFormat function from []cueast.Decl to []Decl in generator_test.go (link)
• Add a unit test for the Struct type and its Build method in decl_test.go (link)

Part of #5364

We may support more CUE types in the future, so we need transparent and extensible cuegen decl.

I have:

• Read and followed KubeVela's contribution process.
• Related Docs updated properly. In a new feature or configuration option, an update to the documentation is necessary.
• Run make reviewable to ensure this PR is ready for review.
• Added backport release-x.y labels to auto-backport this PR if necessary.

How has this code been tested

No break changes and no modified unit tests.

Special notes for your reviewer

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.04 🎉

Comparison is base (da8588c) 60.81% compared to head (b3c4005) 60.86%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6006      +/-   ##
==========================================
+ Coverage   60.81%   60.86%   +0.04%     
==========================================
  Files         224      225       +1     
  Lines       31206    31345     +139     
==========================================
+ Hits        18979    19078      +99     
- Misses      10454    10492      +38     
- Partials     1773     1775       +2     

Flag	Coverage Δ
core-unittests	56.01% <ø> (+0.03%)	⬆️
e2e-multicluster-test	24.93% <ø> (+0.11%)	⬆️
e2etests	25.28% <ø> (-0.01%)	⬇️

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see 21 files with indirect coverage changes

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[Somefive]

/cc @FogDong