feat: Add support for datetime type

[brucechen7274]

What have I done:

1. Enhanced Import Statement Generation Logic: Refactored the import handling of the code generator, which now groups and sorts imports according to Go conventions (standard library, third-party libraries, local libraries).
2. New Datetime Type Support: RFC3339 format.
3. Enhanced Type Validation: Added field type existence validation to ensure referenced types are defined in the specification.

Other things:

If my implementation is okay, do I need to change the date-related fields in the example from string to datetime?

[brucechen7274]

Maybe the parts of generating import code are too complex 🤣

[ostafen]

Left some minor comments. Other than that, looks good!

[ostafen]

I think we can use golang.org/x/tools/imports package to format code. That also handles imports.

Example:

 formattedSrc, err := imports.Process("", src, nil)
    if err != nil {
        log.Fatal(err)
    }

    fmt.Println(string(formattedSrc))

[brucechen7274]

using golang.org/x/tools/imports is a realy eazy way to format our generated code, this makes it completely unnecessary for us to import via a slice of constants.

[ostafen]

Thanks, @brucechen7274!