abigen: create output type for structured contract functions

[joey1123455]

Enhance abigen to Generate Output Types for Contract Functions

Description

Issue #29689

Current limitations:

• It does not create separate output types for contract functions
• This makes it challenging to use the generated bindings in a generic way across different projects

Proposed enhancements:

1. Generate dedicated structs for each function's return type
2. Export these structs in the generated package

Example before:

func SomeFunction() (struct{ Field1 int, Field2 string }, error)

Example after:

type SomeFunctionOutput struct {
    Field1 int
    Field2 string
}

func SomeFunction() (SomeFunctionOutput, error)