abigen: create output type for structured contract functions

[joey1123455]

Enhance abigen to Generate Output Types for Contract Functions

Description

Issue #29689

Current limitations:

• abigen generates Go bindings for Ethereum smart contracts
• 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
3. Update the function signatures to use these new structs instead of anonymous structs

Example before:

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

Example after:

type SomeFunctionOutput struct {
    Field1 int
    Field2 string
}

func SomeFunction() (SomeFunctionOutput, error)