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)

[joey1123455]

@gballet hello, I'm waiting on a review for this pull request

[joey1123455]

@MariusVanDerWijden hello, could you please review my pull request

[fjl]

We will not modify abigen v1 anymore because v2 will be added soon. We will apply this change on abigen v2 when it is ready. The PR can remain open until then.

[joey1123455]

Alright are there any other contributions needed for V2 @fjl so I can pitch in?